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Quasi Experimental Design Overview & Examples

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What is a Quasi Experimental Design?

A quasi experimental design is a method for identifying causal relationships that does not randomly assign participants to the experimental groups. Instead, researchers use a non-random process. For example, they might use an eligibility cutoff score or preexisting groups to determine who receives the treatment.

Quasi-experimental research is a design that closely resembles experimental research but is different. The term “quasi” means “resembling,” so you can think of it as a cousin to actual experiments. In these studies, researchers can manipulate an independent variable — that is, they change one factor to see what effect it has. However, unlike true experimental research, participants are not randomly assigned to different groups.

Learn more about Experimental Designs: Definition & Types .

When to Use Quasi-Experimental Design

Researchers typically use a quasi-experimental design because they can’t randomize due to practical or ethical concerns. For example:

• Practical Constraints : A school interested in testing a new teaching method can only implement it in preexisting classes and cannot randomly assign students.
• Ethical Concerns : A medical study might not be able to randomly assign participants to a treatment group for an experimental medication when they are already taking a proven drug.

Quasi-experimental designs also come in handy when researchers want to study the effects of naturally occurring events, like policy changes or environmental shifts, where they can’t control who is exposed to the treatment.

Quasi-experimental designs occupy a unique position in the spectrum of research methodologies, sitting between observational studies and true experiments. This middle ground offers a blend of both worlds, addressing some limitations of purely observational studies while navigating the constraints often accompanying true experiments.

A significant advantage of quasi-experimental research over purely observational studies and correlational research is that it addresses the issue of directionality, determining which variable is the cause and which is the effect. In quasi-experiments, an intervention typically occurs during the investigation, and the researchers record outcomes before and after it, increasing the confidence that it causes the observed changes.

However, it’s crucial to recognize its limitations as well. Controlling confounding variables is a larger concern for a quasi-experimental design than a true experiment because it lacks random assignment.

In sum, quasi-experimental designs offer a valuable research approach when random assignment is not feasible, providing a more structured and controlled framework than observational studies while acknowledging and attempting to address potential confounders.

Types of Quasi-Experimental Designs and Examples

Quasi-experimental studies use various methods, depending on the scenario.

Natural Experiments

This design uses naturally occurring events or changes to create the treatment and control groups. Researchers compare outcomes between those whom the event affected and those it did not affect. Analysts use statistical controls to account for confounders that the researchers must also measure.

Natural experiments are related to observational studies, but they allow for a clearer causality inference because the external event or policy change provides both a form of quasi-random group assignment and a definite start date for the intervention.

For example, in a natural experiment utilizing a quasi-experimental design, researchers study the impact of a significant economic policy change on small business growth. The policy is implemented in one state but not in neighboring states. This scenario creates an unplanned experimental setup, where the state with the new policy serves as the treatment group, and the neighboring states act as the control group.

Researchers are primarily interested in small business growth rates but need to record various confounders that can impact growth rates. Hence, they record state economic indicators, investment levels, and employment figures. By recording these metrics across the states, they can include them in the model as covariates and control them statistically. This method allows researchers to estimate differences in small business growth due to the policy itself, separate from the various confounders.

Nonequivalent Groups Design

This method involves matching existing groups that are similar but not identical. Researchers attempt to find groups that are as equivalent as possible, particularly for factors likely to affect the outcome.

For instance, researchers use a nonequivalent groups quasi-experimental design to evaluate the effectiveness of a new teaching method in improving students’ mathematics performance. A school district considering the teaching method is planning the study. Students are already divided into schools, preventing random assignment.

The researchers matched two schools with similar demographics, baseline academic performance, and resources. The school using the traditional methodology is the control, while the other uses the new approach. Researchers are evaluating differences in educational outcomes between the two methods.

They perform a pretest to identify differences between the schools that might affect the outcome and include them as covariates to control for confounding. They also record outcomes before and after the intervention to have a larger context for the changes they observe.

Regression Discontinuity

This process assigns subjects to a treatment or control group based on a predetermined cutoff point (e.g., a test score). The analysis primarily focuses on participants near the cutoff point, as they are likely similar except for the treatment received. By comparing participants just above and below the cutoff, the design controls for confounders that vary smoothly around the cutoff.

For example, in a regression discontinuity quasi-experimental design focusing on a new medical treatment for depression, researchers use depression scores as the cutoff point. Individuals with depression scores just above a certain threshold are assigned to receive the latest treatment, while those just below the threshold do not receive it. This method creates two closely matched groups: one that barely qualifies for treatment and one that barely misses out.

By comparing the mental health outcomes of these two groups over time, researchers can assess the effectiveness of the new treatment. The assumption is that the only significant difference between the groups is whether they received the treatment, thereby isolating its impact on depression outcomes.

Controlling Confounders in a Quasi-Experimental Design

Accounting for confounding variables is a challenging but essential task for a quasi-experimental design.

In a true experiment, the random assignment process equalizes confounders across the groups to nullify their overall effect. It’s the gold standard because it works on all confounders, known and unknown.

Unfortunately, the lack of random assignment can allow differences between the groups to exist before the intervention. These confounding factors might ultimately explain the results rather than the intervention.

Consequently, researchers must use other methods to equalize the groups roughly using matching and cutoff values or statistically adjust for preexisting differences they measure to reduce the impact of confounders.

A key strength of quasi-experiments is their frequent use of “pre-post testing.” This approach involves conducting initial tests before collecting data to check for preexisting differences between groups that could impact the study’s outcome. By identifying these variables early on and including them as covariates, researchers can more effectively control potential confounders in their statistical analysis.

Additionally, researchers frequently track outcomes before and after the intervention to better understand the context for changes they observe.

Statisticians consider these methods to be less effective than randomization. Hence, quasi-experiments fall somewhere in the middle when it comes to internal validity , or how well the study can identify causal relationships versus mere correlation . They’re more conclusive than correlational studies but not as solid as true experiments.

In conclusion, quasi-experimental designs offer researchers a versatile and practical approach when random assignment is not feasible. This methodology bridges the gap between controlled experiments and observational studies, providing a valuable tool for investigating cause-and-effect relationships in real-world settings. Researchers can address ethical and logistical constraints by understanding and leveraging the different types of quasi-experimental designs while still obtaining insightful and meaningful results.

Cook, T. D., & Campbell, D. T. (1979).  Quasi-experimentation: Design & analysis issues in field settings . Boston, MA: Houghton Mifflin

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Home » Quasi-Experimental Research Design – Types, Methods

Quasi-Experimental Research Design – Types, Methods

Table of Contents

Quasi-Experimental Design

Quasi-experimental design is a research method that seeks to evaluate the causal relationships between variables, but without the full control over the independent variable(s) that is available in a true experimental design.

In a quasi-experimental design, the researcher uses an existing group of participants that is not randomly assigned to the experimental and control groups. Instead, the groups are selected based on pre-existing characteristics or conditions, such as age, gender, or the presence of a certain medical condition.

Types of Quasi-Experimental Design

There are several types of quasi-experimental designs that researchers use to study causal relationships between variables. Here are some of the most common types:

Non-Equivalent Control Group Design

This design involves selecting two groups of participants that are similar in every way except for the independent variable(s) that the researcher is testing. One group receives the treatment or intervention being studied, while the other group does not. The two groups are then compared to see if there are any significant differences in the outcomes.

Interrupted Time-Series Design

This design involves collecting data on the dependent variable(s) over a period of time, both before and after an intervention or event. The researcher can then determine whether there was a significant change in the dependent variable(s) following the intervention or event.

Pretest-Posttest Design

This design involves measuring the dependent variable(s) before and after an intervention or event, but without a control group. This design can be useful for determining whether the intervention or event had an effect, but it does not allow for control over other factors that may have influenced the outcomes.

Regression Discontinuity Design

This design involves selecting participants based on a specific cutoff point on a continuous variable, such as a test score. Participants on either side of the cutoff point are then compared to determine whether the intervention or event had an effect.

Natural Experiments

This design involves studying the effects of an intervention or event that occurs naturally, without the researcher’s intervention. For example, a researcher might study the effects of a new law or policy that affects certain groups of people. This design is useful when true experiments are not feasible or ethical.

Data Analysis Methods

Here are some data analysis methods that are commonly used in quasi-experimental designs:

Descriptive Statistics

This method involves summarizing the data collected during a study using measures such as mean, median, mode, range, and standard deviation. Descriptive statistics can help researchers identify trends or patterns in the data, and can also be useful for identifying outliers or anomalies.

Inferential Statistics

This method involves using statistical tests to determine whether the results of a study are statistically significant. Inferential statistics can help researchers make generalizations about a population based on the sample data collected during the study. Common statistical tests used in quasi-experimental designs include t-tests, ANOVA, and regression analysis.

Propensity Score Matching

This method is used to reduce bias in quasi-experimental designs by matching participants in the intervention group with participants in the control group who have similar characteristics. This can help to reduce the impact of confounding variables that may affect the study’s results.

Difference-in-differences Analysis

This method is used to compare the difference in outcomes between two groups over time. Researchers can use this method to determine whether a particular intervention has had an impact on the target population over time.

Interrupted Time Series Analysis

This method is used to examine the impact of an intervention or treatment over time by comparing data collected before and after the intervention or treatment. This method can help researchers determine whether an intervention had a significant impact on the target population.

Regression Discontinuity Analysis

This method is used to compare the outcomes of participants who fall on either side of a predetermined cutoff point. This method can help researchers determine whether an intervention had a significant impact on the target population.

Steps in Quasi-Experimental Design

Here are the general steps involved in conducting a quasi-experimental design:

• Identify the research question: Determine the research question and the variables that will be investigated.
• Choose the design: Choose the appropriate quasi-experimental design to address the research question. Examples include the pretest-posttest design, non-equivalent control group design, regression discontinuity design, and interrupted time series design.
• Select the participants: Select the participants who will be included in the study. Participants should be selected based on specific criteria relevant to the research question.
• Measure the variables: Measure the variables that are relevant to the research question. This may involve using surveys, questionnaires, tests, or other measures.
• Implement the intervention or treatment: Implement the intervention or treatment to the participants in the intervention group. This may involve training, education, counseling, or other interventions.
• Collect data: Collect data on the dependent variable(s) before and after the intervention. Data collection may also include collecting data on other variables that may impact the dependent variable(s).
• Analyze the data: Analyze the data collected to determine whether the intervention had a significant impact on the dependent variable(s).
• Draw conclusions: Draw conclusions about the relationship between the independent and dependent variables. If the results suggest a causal relationship, then appropriate recommendations may be made based on the findings.

Quasi-Experimental Design Examples

Here are some examples of real-time quasi-experimental designs:

• Evaluating the impact of a new teaching method: In this study, a group of students are taught using a new teaching method, while another group is taught using the traditional method. The test scores of both groups are compared before and after the intervention to determine whether the new teaching method had a significant impact on student performance.
• Assessing the effectiveness of a public health campaign: In this study, a public health campaign is launched to promote healthy eating habits among a targeted population. The behavior of the population is compared before and after the campaign to determine whether the intervention had a significant impact on the target behavior.
• Examining the impact of a new medication: In this study, a group of patients is given a new medication, while another group is given a placebo. The outcomes of both groups are compared to determine whether the new medication had a significant impact on the targeted health condition.
• Evaluating the effectiveness of a job training program : In this study, a group of unemployed individuals is enrolled in a job training program, while another group is not enrolled in any program. The employment rates of both groups are compared before and after the intervention to determine whether the training program had a significant impact on the employment rates of the participants.
• Assessing the impact of a new policy : In this study, a new policy is implemented in a particular area, while another area does not have the new policy. The outcomes of both areas are compared before and after the intervention to determine whether the new policy had a significant impact on the targeted behavior or outcome.

Applications of Quasi-Experimental Design

Here are some applications of quasi-experimental design:

• Educational research: Quasi-experimental designs are used to evaluate the effectiveness of educational interventions, such as new teaching methods, technology-based learning, or educational policies.
• Health research: Quasi-experimental designs are used to evaluate the effectiveness of health interventions, such as new medications, public health campaigns, or health policies.
• Social science research: Quasi-experimental designs are used to investigate the impact of social interventions, such as job training programs, welfare policies, or criminal justice programs.
• Business research: Quasi-experimental designs are used to evaluate the impact of business interventions, such as marketing campaigns, new products, or pricing strategies.
• Environmental research: Quasi-experimental designs are used to evaluate the impact of environmental interventions, such as conservation programs, pollution control policies, or renewable energy initiatives.

When to use Quasi-Experimental Design

Here are some situations where quasi-experimental designs may be appropriate:

• When the research question involves investigating the effectiveness of an intervention, policy, or program : In situations where it is not feasible or ethical to randomly assign participants to intervention and control groups, quasi-experimental designs can be used to evaluate the impact of the intervention on the targeted outcome.
• When the sample size is small: In situations where the sample size is small, it may be difficult to randomly assign participants to intervention and control groups. Quasi-experimental designs can be used to investigate the impact of an intervention without requiring a large sample size.
• When the research question involves investigating a naturally occurring event : In some situations, researchers may be interested in investigating the impact of a naturally occurring event, such as a natural disaster or a major policy change. Quasi-experimental designs can be used to evaluate the impact of the event on the targeted outcome.
• When the research question involves investigating a long-term intervention: In situations where the intervention or program is long-term, it may be difficult to randomly assign participants to intervention and control groups for the entire duration of the intervention. Quasi-experimental designs can be used to evaluate the impact of the intervention over time.
• When the research question involves investigating the impact of a variable that cannot be manipulated : In some situations, it may not be possible or ethical to manipulate a variable of interest. Quasi-experimental designs can be used to investigate the relationship between the variable and the targeted outcome.

Purpose of Quasi-Experimental Design

The purpose of quasi-experimental design is to investigate the causal relationship between two or more variables when it is not feasible or ethical to conduct a randomized controlled trial (RCT). Quasi-experimental designs attempt to emulate the randomized control trial by mimicking the control group and the intervention group as much as possible.

The key purpose of quasi-experimental design is to evaluate the impact of an intervention, policy, or program on a targeted outcome while controlling for potential confounding factors that may affect the outcome. Quasi-experimental designs aim to answer questions such as: Did the intervention cause the change in the outcome? Would the outcome have changed without the intervention? And was the intervention effective in achieving its intended goals?

Quasi-experimental designs are useful in situations where randomized controlled trials are not feasible or ethical. They provide researchers with an alternative method to evaluate the effectiveness of interventions, policies, and programs in real-life settings. Quasi-experimental designs can also help inform policy and practice by providing valuable insights into the causal relationships between variables.

Overall, the purpose of quasi-experimental design is to provide a rigorous method for evaluating the impact of interventions, policies, and programs while controlling for potential confounding factors that may affect the outcome.

Advantages of Quasi-Experimental Design

Quasi-experimental designs have several advantages over other research designs, such as:

• Greater external validity : Quasi-experimental designs are more likely to have greater external validity than laboratory experiments because they are conducted in naturalistic settings. This means that the results are more likely to generalize to real-world situations.
• Ethical considerations: Quasi-experimental designs often involve naturally occurring events, such as natural disasters or policy changes. This means that researchers do not need to manipulate variables, which can raise ethical concerns.
• More practical: Quasi-experimental designs are often more practical than experimental designs because they are less expensive and easier to conduct. They can also be used to evaluate programs or policies that have already been implemented, which can save time and resources.
• No random assignment: Quasi-experimental designs do not require random assignment, which can be difficult or impossible in some cases, such as when studying the effects of a natural disaster. This means that researchers can still make causal inferences, although they must use statistical techniques to control for potential confounding variables.
• Greater generalizability : Quasi-experimental designs are often more generalizable than experimental designs because they include a wider range of participants and conditions. This can make the results more applicable to different populations and settings.

Limitations of Quasi-Experimental Design

There are several limitations associated with quasi-experimental designs, which include:

• Lack of Randomization: Quasi-experimental designs do not involve randomization of participants into groups, which means that the groups being studied may differ in important ways that could affect the outcome of the study. This can lead to problems with internal validity and limit the ability to make causal inferences.
• Selection Bias: Quasi-experimental designs may suffer from selection bias because participants are not randomly assigned to groups. Participants may self-select into groups or be assigned based on pre-existing characteristics, which may introduce bias into the study.
• History and Maturation: Quasi-experimental designs are susceptible to history and maturation effects, where the passage of time or other events may influence the outcome of the study.
• Lack of Control: Quasi-experimental designs may lack control over extraneous variables that could influence the outcome of the study. This can limit the ability to draw causal inferences from the study.
• Limited Generalizability: Quasi-experimental designs may have limited generalizability because the results may only apply to the specific population and context being studied.

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Experimental vs Quasi-Experimental Design: Which to Choose?

Here’s a table that summarizes the similarities and differences between an experimental and a quasi-experimental study design:

What is a quasi-experimental design?

A quasi-experimental design is a non-randomized study design used to evaluate the effect of an intervention. The intervention can be a training program, a policy change or a medical treatment.

Unlike a true experiment, in a quasi-experimental study the choice of who gets the intervention and who doesn’t is not randomized. Instead, the intervention can be assigned to participants according to their choosing or that of the researcher, or by using any method other than randomness.

Having a control group is not required, but if present, it provides a higher level of evidence for the relationship between the intervention and the outcome.

(for more information, I recommend my other article: Understand Quasi-Experimental Design Through an Example ) .

Examples of quasi-experimental designs include:

• One-Group Posttest Only Design
• Static-Group Comparison Design
• One-Group Pretest-Posttest Design
• Separate-Sample Pretest-Posttest Design

What is an experimental design?

An experimental design is a randomized study design used to evaluate the effect of an intervention. In its simplest form, the participants will be randomly divided into 2 groups:

• A treatment group: where participants receive the new intervention which effect we want to study.
• A control or comparison group: where participants do not receive any intervention at all (or receive some standard intervention).

Randomization ensures that each participant has the same chance of receiving the intervention. Its objective is to equalize the 2 groups, and therefore, any observed difference in the study outcome afterwards will only be attributed to the intervention – i.e. it removes confounding.

(for more information, I recommend my other article: Purpose and Limitations of Random Assignment ).

Examples of experimental designs include:

• Posttest-Only Control Group Design
• Pretest-Posttest Control Group Design
• Solomon Four-Group Design
• Matched Pairs Design
• Randomized Block Design

When to choose an experimental design over a quasi-experimental design?

Although many statistical techniques can be used to deal with confounding in a quasi-experimental study, in practice, randomization is still the best tool we have to study causal relationships.

Another problem with quasi-experiments is the natural progression of the disease or the condition under study — When studying the effect of an intervention over time, one should consider natural changes because these can be mistaken with changes in outcome that are caused by the intervention. Having a well-chosen control group helps dealing with this issue.

So, if losing the element of randomness seems like an unwise step down in the hierarchy of evidence, why would we ever want to do it?

This is what we’re going to discuss next.

When to choose a quasi-experimental design over a true experiment?

The issue with randomness is that it cannot be always achievable.

So here are some cases where using a quasi-experimental design makes more sense than using an experimental one:

• If being in one group is believed to be harmful for the participants , either because the intervention is harmful (ex. randomizing people to smoking), or the intervention has a questionable efficacy, or on the contrary it is believed to be so beneficial that it would be malevolent to put people in the control group (ex. randomizing people to receiving an operation).
• In cases where interventions act on a group of people in a given location , it becomes difficult to adequately randomize subjects (ex. an intervention that reduces pollution in a given area).
• When working with small sample sizes , as randomized controlled trials require a large sample size to account for heterogeneity among subjects (i.e. to evenly distribute confounding variables between the intervention and control groups).

Further reading

• Statistical Software Popularity in 40,582 Research Papers
• Checking the Popularity of 125 Statistical Tests and Models
• Objectives of Epidemiology (With Examples)
• 12 Famous Epidemiologists and Why

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• > The Cambridge Handbook of Research Methods and Statistics for the Social and Behavioral Sciences
• > Quasi-Experimental Research

Book contents

• The Cambridge Handbook of Research Methods and Statistics for the Social and Behavioral Sciences
• Cambridge Handbooks in Psychology
• Copyright page
• Contributors
• Part I From Idea to Reality: The Basics of Research
• Part II The Building Blocks of a Study
• Part III Data Collection
• 13 Cross-Sectional Studies
• 14 Quasi-Experimental Research
• 15 Non-equivalent Control Group Pretest–Posttest Design in Social and Behavioral Research
• 16 Experimental Methods
• 17 Longitudinal Research: A World to Explore
• 18 Online Research Methods
• 19 Archival Data
• 20 Qualitative Research Design
• Part IV Statistical Approaches
• Part V Tips for a Successful Research Career

14 - Quasi-Experimental Research

from Part III - Data Collection

Published online by Cambridge University Press:  25 May 2023

In this chapter, we discuss the logic and practice of quasi-experimentation. Specifically, we describe four quasi-experimental designs – one-group pretest–posttest designs, non-equivalent group designs, regression discontinuity designs, and interrupted time-series designs – and their statistical analyses in detail. Both simple quasi-experimental designs and embellishments of these simple designs are presented. Potential threats to internal validity are illustrated along with means of addressing their potentially biasing effects so that these effects can be minimized. In contrast to quasi-experiments, randomized experiments are often thought to be the gold standard when estimating the effects of treatment interventions. However, circumstances frequently arise where quasi-experiments can usefully supplement randomized experiments or when quasi-experiments can fruitfully be used in place of randomized experiments. Researchers need to appreciate the relative strengths and weaknesses of the various quasi-experiments so they can choose among pre-specified designs or craft their own unique quasi-experiments.

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• Quasi-Experimental Research
• By Charles S. Reichardt , Daniel Storage , Damon Abraham
• Edited by Austin Lee Nichols , Central European University, Vienna , John Edlund , Rochester Institute of Technology, New York
• Book: The Cambridge Handbook of Research Methods and Statistics for the Social and Behavioral Sciences
• Online publication: 25 May 2023
• Chapter DOI: https://doi.org/10.1017/9781009010054.015

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7.3 Quasi-Experimental Research

Learning objectives.

• Explain what quasi-experimental research is and distinguish it clearly from both experimental and correlational research.
• Describe three different types of quasi-experimental research designs (nonequivalent groups, pretest-posttest, and interrupted time series) and identify examples of each one.

The prefix quasi means “resembling.” Thus quasi-experimental research is research that resembles experimental research but is not true experimental research. Although the independent variable is manipulated, participants are not randomly assigned to conditions or orders of conditions (Cook & Campbell, 1979). Because the independent variable is manipulated before the dependent variable is measured, quasi-experimental research eliminates the directionality problem. But because participants are not randomly assigned—making it likely that there are other differences between conditions—quasi-experimental research does not eliminate the problem of confounding variables. In terms of internal validity, therefore, quasi-experiments are generally somewhere between correlational studies and true experiments.

Quasi-experiments are most likely to be conducted in field settings in which random assignment is difficult or impossible. They are often conducted to evaluate the effectiveness of a treatment—perhaps a type of psychotherapy or an educational intervention. There are many different kinds of quasi-experiments, but we will discuss just a few of the most common ones here.

Nonequivalent Groups Design

Recall that when participants in a between-subjects experiment are randomly assigned to conditions, the resulting groups are likely to be quite similar. In fact, researchers consider them to be equivalent. When participants are not randomly assigned to conditions, however, the resulting groups are likely to be dissimilar in some ways. For this reason, researchers consider them to be nonequivalent. A nonequivalent groups design , then, is a between-subjects design in which participants have not been randomly assigned to conditions.

Imagine, for example, a researcher who wants to evaluate a new method of teaching fractions to third graders. One way would be to conduct a study with a treatment group consisting of one class of third-grade students and a control group consisting of another class of third-grade students. This would be a nonequivalent groups design because the students are not randomly assigned to classes by the researcher, which means there could be important differences between them. For example, the parents of higher achieving or more motivated students might have been more likely to request that their children be assigned to Ms. Williams’s class. Or the principal might have assigned the “troublemakers” to Mr. Jones’s class because he is a stronger disciplinarian. Of course, the teachers’ styles, and even the classroom environments, might be very different and might cause different levels of achievement or motivation among the students. If at the end of the study there was a difference in the two classes’ knowledge of fractions, it might have been caused by the difference between the teaching methods—but it might have been caused by any of these confounding variables.

Of course, researchers using a nonequivalent groups design can take steps to ensure that their groups are as similar as possible. In the present example, the researcher could try to select two classes at the same school, where the students in the two classes have similar scores on a standardized math test and the teachers are the same sex, are close in age, and have similar teaching styles. Taking such steps would increase the internal validity of the study because it would eliminate some of the most important confounding variables. But without true random assignment of the students to conditions, there remains the possibility of other important confounding variables that the researcher was not able to control.

Pretest-Posttest Design

In a pretest-posttest design , the dependent variable is measured once before the treatment is implemented and once after it is implemented. Imagine, for example, a researcher who is interested in the effectiveness of an antidrug education program on elementary school students’ attitudes toward illegal drugs. The researcher could measure the attitudes of students at a particular elementary school during one week, implement the antidrug program during the next week, and finally, measure their attitudes again the following week. The pretest-posttest design is much like a within-subjects experiment in which each participant is tested first under the control condition and then under the treatment condition. It is unlike a within-subjects experiment, however, in that the order of conditions is not counterbalanced because it typically is not possible for a participant to be tested in the treatment condition first and then in an “untreated” control condition.

If the average posttest score is better than the average pretest score, then it makes sense to conclude that the treatment might be responsible for the improvement. Unfortunately, one often cannot conclude this with a high degree of certainty because there may be other explanations for why the posttest scores are better. One category of alternative explanations goes under the name of history . Other things might have happened between the pretest and the posttest. Perhaps an antidrug program aired on television and many of the students watched it, or perhaps a celebrity died of a drug overdose and many of the students heard about it. Another category of alternative explanations goes under the name of maturation . Participants might have changed between the pretest and the posttest in ways that they were going to anyway because they are growing and learning. If it were a yearlong program, participants might become less impulsive or better reasoners and this might be responsible for the change.

Another alternative explanation for a change in the dependent variable in a pretest-posttest design is regression to the mean . This refers to the statistical fact that an individual who scores extremely on a variable on one occasion will tend to score less extremely on the next occasion. For example, a bowler with a long-term average of 150 who suddenly bowls a 220 will almost certainly score lower in the next game. Her score will “regress” toward her mean score of 150. Regression to the mean can be a problem when participants are selected for further study because of their extreme scores. Imagine, for example, that only students who scored especially low on a test of fractions are given a special training program and then retested. Regression to the mean all but guarantees that their scores will be higher even if the training program has no effect. A closely related concept—and an extremely important one in psychological research—is spontaneous remission . This is the tendency for many medical and psychological problems to improve over time without any form of treatment. The common cold is a good example. If one were to measure symptom severity in 100 common cold sufferers today, give them a bowl of chicken soup every day, and then measure their symptom severity again in a week, they would probably be much improved. This does not mean that the chicken soup was responsible for the improvement, however, because they would have been much improved without any treatment at all. The same is true of many psychological problems. A group of severely depressed people today is likely to be less depressed on average in 6 months. In reviewing the results of several studies of treatments for depression, researchers Michael Posternak and Ivan Miller found that participants in waitlist control conditions improved an average of 10 to 15% before they received any treatment at all (Posternak & Miller, 2001). Thus one must generally be very cautious about inferring causality from pretest-posttest designs.

Does Psychotherapy Work?

Early studies on the effectiveness of psychotherapy tended to use pretest-posttest designs. In a classic 1952 article, researcher Hans Eysenck summarized the results of 24 such studies showing that about two thirds of patients improved between the pretest and the posttest (Eysenck, 1952). But Eysenck also compared these results with archival data from state hospital and insurance company records showing that similar patients recovered at about the same rate without receiving psychotherapy. This suggested to Eysenck that the improvement that patients showed in the pretest-posttest studies might be no more than spontaneous remission. Note that Eysenck did not conclude that psychotherapy was ineffective. He merely concluded that there was no evidence that it was, and he wrote of “the necessity of properly planned and executed experimental studies into this important field” (p. 323). You can read the entire article here:

http://psychclassics.yorku.ca/Eysenck/psychotherapy.htm

Fortunately, many other researchers took up Eysenck’s challenge, and by 1980 hundreds of experiments had been conducted in which participants were randomly assigned to treatment and control conditions, and the results were summarized in a classic book by Mary Lee Smith, Gene Glass, and Thomas Miller (Smith, Glass, & Miller, 1980). They found that overall psychotherapy was quite effective, with about 80% of treatment participants improving more than the average control participant. Subsequent research has focused more on the conditions under which different types of psychotherapy are more or less effective.

In a classic 1952 article, researcher Hans Eysenck pointed out the shortcomings of the simple pretest-posttest design for evaluating the effectiveness of psychotherapy.

Wikimedia Commons – CC BY-SA 3.0.

Interrupted Time Series Design

A variant of the pretest-posttest design is the interrupted time-series design . A time series is a set of measurements taken at intervals over a period of time. For example, a manufacturing company might measure its workers’ productivity each week for a year. In an interrupted time series-design, a time series like this is “interrupted” by a treatment. In one classic example, the treatment was the reduction of the work shifts in a factory from 10 hours to 8 hours (Cook & Campbell, 1979). Because productivity increased rather quickly after the shortening of the work shifts, and because it remained elevated for many months afterward, the researcher concluded that the shortening of the shifts caused the increase in productivity. Notice that the interrupted time-series design is like a pretest-posttest design in that it includes measurements of the dependent variable both before and after the treatment. It is unlike the pretest-posttest design, however, in that it includes multiple pretest and posttest measurements.

Figure 7.5 “A Hypothetical Interrupted Time-Series Design” shows data from a hypothetical interrupted time-series study. The dependent variable is the number of student absences per week in a research methods course. The treatment is that the instructor begins publicly taking attendance each day so that students know that the instructor is aware of who is present and who is absent. The top panel of Figure 7.5 “A Hypothetical Interrupted Time-Series Design” shows how the data might look if this treatment worked. There is a consistently high number of absences before the treatment, and there is an immediate and sustained drop in absences after the treatment. The bottom panel of Figure 7.5 “A Hypothetical Interrupted Time-Series Design” shows how the data might look if this treatment did not work. On average, the number of absences after the treatment is about the same as the number before. This figure also illustrates an advantage of the interrupted time-series design over a simpler pretest-posttest design. If there had been only one measurement of absences before the treatment at Week 7 and one afterward at Week 8, then it would have looked as though the treatment were responsible for the reduction. The multiple measurements both before and after the treatment suggest that the reduction between Weeks 7 and 8 is nothing more than normal week-to-week variation.

Figure 7.5 A Hypothetical Interrupted Time-Series Design

The top panel shows data that suggest that the treatment caused a reduction in absences. The bottom panel shows data that suggest that it did not.

Combination Designs

A type of quasi-experimental design that is generally better than either the nonequivalent groups design or the pretest-posttest design is one that combines elements of both. There is a treatment group that is given a pretest, receives a treatment, and then is given a posttest. But at the same time there is a control group that is given a pretest, does not receive the treatment, and then is given a posttest. The question, then, is not simply whether participants who receive the treatment improve but whether they improve more than participants who do not receive the treatment.

Imagine, for example, that students in one school are given a pretest on their attitudes toward drugs, then are exposed to an antidrug program, and finally are given a posttest. Students in a similar school are given the pretest, not exposed to an antidrug program, and finally are given a posttest. Again, if students in the treatment condition become more negative toward drugs, this could be an effect of the treatment, but it could also be a matter of history or maturation. If it really is an effect of the treatment, then students in the treatment condition should become more negative than students in the control condition. But if it is a matter of history (e.g., news of a celebrity drug overdose) or maturation (e.g., improved reasoning), then students in the two conditions would be likely to show similar amounts of change. This type of design does not completely eliminate the possibility of confounding variables, however. Something could occur at one of the schools but not the other (e.g., a student drug overdose), so students at the first school would be affected by it while students at the other school would not.

Finally, if participants in this kind of design are randomly assigned to conditions, it becomes a true experiment rather than a quasi experiment. In fact, it is the kind of experiment that Eysenck called for—and that has now been conducted many times—to demonstrate the effectiveness of psychotherapy.

Key Takeaways

• Quasi-experimental research involves the manipulation of an independent variable without the random assignment of participants to conditions or orders of conditions. Among the important types are nonequivalent groups designs, pretest-posttest, and interrupted time-series designs.
• Quasi-experimental research eliminates the directionality problem because it involves the manipulation of the independent variable. It does not eliminate the problem of confounding variables, however, because it does not involve random assignment to conditions. For these reasons, quasi-experimental research is generally higher in internal validity than correlational studies but lower than true experiments.
• Practice: Imagine that two college professors decide to test the effect of giving daily quizzes on student performance in a statistics course. They decide that Professor A will give quizzes but Professor B will not. They will then compare the performance of students in their two sections on a common final exam. List five other variables that might differ between the two sections that could affect the results.

Discussion: Imagine that a group of obese children is recruited for a study in which their weight is measured, then they participate for 3 months in a program that encourages them to be more active, and finally their weight is measured again. Explain how each of the following might affect the results:

• regression to the mean
• spontaneous remission

Cook, T. D., & Campbell, D. T. (1979). Quasi-experimentation: Design & analysis issues in field settings . Boston, MA: Houghton Mifflin.

Eysenck, H. J. (1952). The effects of psychotherapy: An evaluation. Journal of Consulting Psychology, 16 , 319–324.

Posternak, M. A., & Miller, I. (2001). Untreated short-term course of major depression: A meta-analysis of studies using outcomes from studies using wait-list control groups. Journal of Affective Disorders, 66 , 139–146.

Smith, M. L., Glass, G. V., & Miller, T. I. (1980). The benefits of psychotherapy . Baltimore, MD: Johns Hopkins University Press.

Research Methods in Psychology Copyright © 2016 by University of Minnesota is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Quasi-Experimental Design

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Quasi-Experimental Design is a unique research methodology because it is characterized by what is lacks. For example, Abraham & MacDonald (2011) state:

" Quasi-experimental research is similar to experimental research in that there is manipulation of an independent variable. It differs from experimental research because either there is no control group, no random selection, no random assignment, and/or no active manipulation. "

This type of research is often performed in cases where a control group cannot be created or random selection cannot be performed. This is often the case in certain medical and psychological studies. 

For more information on quasi-experimental design, review the resources below: 

Where to Start

Below are listed a few tools and online guides that can help you start your Quasi-experimental research. These include free online resources and resources available only through ISU Library.

• Quasi-Experimental Research Designs by Bruce A. Thyer This pocket guide describes the logic, design, and conduct of the range of quasi-experimental designs, encompassing pre-experiments, quasi-experiments making use of a control or comparison group, and time-series designs. An introductory chapter describes the valuable role these types of studies have played in social work, from the 1930s to the present. Subsequent chapters delve into each design type's major features, the kinds of questions it is capable of answering, and its strengths and limitations.
• Experimental and Quasi-Experimental Designs for Research by Donald T. Campbell; Julian C. Stanley. Call Number: Q175 C152e Written 1967 but still used heavily today, this book examines research designs for experimental and quasi-experimental research, with examples and judgments about each design's validity.

Online Resources

• Quasi-Experimental Design From the Web Center for Social Research Methods, this is a very good overview of quasi-experimental design.
• Experimental and Quasi-Experimental Research From Colorado State University.
• Quasi-experimental design--Wikipedia, the free encyclopedia Wikipedia can be a useful place to start your research- check the citations at the bottom of the article for more information.
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Quasi-experimental Research: What It Is, Types & Examples

Much like an actual experiment, quasi-experimental research tries to demonstrate a cause-and-effect link between a dependent and an independent variable. A quasi-experiment, on the other hand, does not depend on random assignment, unlike an actual experiment. The subjects are sorted into groups based on non-random variables.

What is Quasi-Experimental Research?

“Resemblance” is the definition of “quasi.” Individuals are not randomly allocated to conditions or orders of conditions, even though the regression analysis is changed. As a result, quasi-experimental research is research that appears to be experimental but is not.

The directionality problem is avoided in quasi-experimental research since the regression analysis is altered before the multiple regression is assessed. However, because individuals are not randomized at random, there are likely to be additional disparities across conditions in quasi-experimental research.

As a result, in terms of internal consistency, quasi-experiments fall somewhere between correlational research and actual experiments.

The key component of a true experiment is randomly allocated groups. This means that each person has an equivalent chance of being assigned to the experimental group or the control group, depending on whether they are manipulated or not.

Simply put, a quasi-experiment is not a real experiment. A quasi-experiment does not feature randomly allocated groups since the main component of a real experiment is randomly assigned groups. Why is it so crucial to have randomly allocated groups, given that they constitute the only distinction between quasi-experimental and actual  experimental research ?

Let’s use an example to illustrate our point. Let’s assume we want to discover how new psychological therapy affects depressed patients. In a genuine trial, you’d split half of the psych ward into treatment groups, With half getting the new psychotherapy therapy and the other half receiving standard  depression treatment .

And the physicians compare the outcomes of this treatment to the results of standard treatments to see if this treatment is more effective. Doctors, on the other hand, are unlikely to agree with this genuine experiment since they believe it is unethical to treat one group while leaving another untreated.

A quasi-experimental study will be useful in this case. Instead of allocating these patients at random, you uncover pre-existing psychotherapist groups in the hospitals. Clearly, there’ll be counselors who are eager to undertake these trials as well as others who prefer to stick to the old ways.

These pre-existing groups can be used to compare the symptom development of individuals who received the novel therapy with those who received the normal course of treatment, even though the groups weren’t chosen at random.

If any substantial variations between them can be well explained, you may be very assured that any differences are attributable to the treatment but not to other extraneous variables.

As we mentioned before, quasi-experimental research entails manipulating an independent variable by randomly assigning people to conditions or sequences of conditions. Non-equivalent group designs, pretest-posttest designs, and regression discontinuity designs are only a few of the essential types.

What are quasi-experimental research designs?

Quasi-experimental research designs are a type of research design that is similar to experimental designs but doesn’t give full control over the independent variable(s) like true experimental designs do.

In a quasi-experimental design, the researcher changes or watches an independent variable, but the participants are not put into groups at random. Instead, people are put into groups based on things they already have in common, like their age, gender, or how many times they have seen a certain stimulus.

Because the assignments are not random, it is harder to draw conclusions about cause and effect than in a real experiment. However, quasi-experimental designs are still useful when randomization is not possible or ethical.

The true experimental design may be impossible to accomplish or just too expensive, especially for researchers with few resources. Quasi-experimental designs enable you to investigate an issue by utilizing data that has already been paid for or gathered by others (often the government). 

Because they allow better control for confounding variables than other forms of studies, they have higher external validity than most genuine experiments and higher  internal validity  (less than true experiments) than other non-experimental research.

Is quasi-experimental research quantitative or qualitative?

Quasi-experimental research is a quantitative research method. It involves numerical data collection and statistical analysis. Quasi-experimental research compares groups with different circumstances or treatments to find cause-and-effect links. 

It draws statistical conclusions from quantitative data. Qualitative data can enhance quasi-experimental research by revealing participants’ experiences and opinions, but quantitative data is the method’s foundation.

Quasi-experimental research types

There are many different sorts of quasi-experimental designs. Three of the most popular varieties are described below: Design of non-equivalent groups, Discontinuity in regression, and Natural experiments.

Design of Non-equivalent Groups

Example: design of non-equivalent groups, discontinuity in regression, example: discontinuity in regression, natural experiments, example: natural experiments.

However, because they couldn’t afford to pay everyone who qualified for the program, they had to use a random lottery to distribute slots.

Experts were able to investigate the program’s impact by utilizing enrolled people as a treatment group and those who were qualified but did not play the jackpot as an experimental group.

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Experimental and Quasi-Experimental Methods

• Reference work entry
• First Online: 01 January 2014
• Cite this reference work entry

• Roger J. R. Levesque 2  

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Research designs are central to research projects in that they constitute the projects’ basic structure that will permit researchers to address their main research questions. Designs include, for example, the selection of relevant samples or groups, measures, treatments or programs, and methods of assignment. The two key designs that help researchers address whether a program or treatment causes an outcome are the experimental design, which uses random assignment to groups or programs, and quasi-experimental designs, which do not use random assignment (see Shadish et al. 2002 ; Bell 2010 ; Trochim 2006 ). These two methods are important to consider in that even the experimental design may not prove causation, and causation is what researchers often aim to show when they analyze data (e.g., they try to show that an outcome is likely to follow given a certain set of conditions). Still, the general rule tends to be that studies unable to determine causality are classified as...

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Bell, S. H. (2010). The urban institute research of record: Quasi-experimental methods. Washington, DC: The Urban Institute. Retrieved Nov. 20, 2010, from http://www.urban.org/toolkit/data-methods/quasi-experimental.cfm

Campbell, D. T., & Stanley, J. C. (1966). Experimental and quasi-experimental designs for research . Chicago: Rand McNally.

Google Scholar  

Harris, A. D., McGregor, J. C., Perencevich, E. N., Furuno, J. P., Zhu, J., Peterson, D. E., & Finkelstein, J. (2006). The use and interpretation of quasi-experimental studies in medical informatics. The Journal of American Medical Informatics Association, 13 , 16–23.

Shadish, W. R., Cook, T. D., & Campbell, T. D. (2002). Experimental and quasi-experimental designs for generalized causal inference . Boston: Houghton-Mifflin.

Trochim, W. M. (2006). The research methods knowledge base (2nd ed.). Cincinnati: Atomic Dog. Retrieved Nov. 20, 2011, from http://www.socialresearchmethods.net/kb/

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Levesque, R.J.R. (2011). Experimental and Quasi-Experimental Methods. In: Levesque, R.J.R. (eds) Encyclopedia of Adolescence. Springer, New York, NY. https://doi.org/10.1007/978-1-4419-1695-2_655

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The Use and Interpretation of Quasi-Experimental Studies in Medical Informatics

Associated data.

Quasi-experimental study designs, often described as nonrandomized, pre-post intervention studies, are common in the medical informatics literature. Yet little has been written about the benefits and limitations of the quasi-experimental approach as applied to informatics studies. This paper outlines a relative hierarchy and nomenclature of quasi-experimental study designs that is applicable to medical informatics intervention studies. In addition, the authors performed a systematic review of two medical informatics journals, the Journal of the American Medical Informatics Association (JAMIA) and the International Journal of Medical Informatics (IJMI), to determine the number of quasi-experimental studies published and how the studies are classified on the above-mentioned relative hierarchy. They hope that future medical informatics studies will implement higher level quasi-experimental study designs that yield more convincing evidence for causal links between medical informatics interventions and outcomes.

Quasi-experimental studies encompass a broad range of nonrandomized intervention studies. These designs are frequently used when it is not logistically feasible or ethical to conduct a randomized controlled trial. Examples of quasi-experimental studies follow. As one example of a quasi-experimental study, a hospital introduces a new order-entry system and wishes to study the impact of this intervention on the number of medication-related adverse events before and after the intervention. As another example, an informatics technology group is introducing a pharmacy order-entry system aimed at decreasing pharmacy costs. The intervention is implemented and pharmacy costs before and after the intervention are measured.

In medical informatics, the quasi-experimental, sometimes called the pre-post intervention, design often is used to evaluate the benefits of specific interventions. The increasing capacity of health care institutions to collect routine clinical data has led to the growing use of quasi-experimental study designs in the field of medical informatics as well as in other medical disciplines. However, little is written about these study designs in the medical literature or in traditional epidemiology textbooks. 1 , 2 , 3 In contrast, the social sciences literature is replete with examples of ways to implement and improve quasi-experimental studies. 4 , 5 , 6

In this paper, we review the different pretest-posttest quasi-experimental study designs, their nomenclature, and the relative hierarchy of these designs with respect to their ability to establish causal associations between an intervention and an outcome. The example of a pharmacy order-entry system aimed at decreasing pharmacy costs will be used throughout this article to illustrate the different quasi-experimental designs. We discuss limitations of quasi-experimental designs and offer methods to improve them. We also perform a systematic review of four years of publications from two informatics journals to determine the number of quasi-experimental studies, classify these studies into their application domains, determine whether the potential limitations of quasi-experimental studies were acknowledged by the authors, and place these studies into the above-mentioned relative hierarchy.

The authors reviewed articles and book chapters on the design of quasi-experimental studies. 4 , 5 , 6 , 7 , 8 , 9 , 10 Most of the reviewed articles referenced two textbooks that were then reviewed in depth. 4 , 6

Key advantages and disadvantages of quasi-experimental studies, as they pertain to the study of medical informatics, were identified. The potential methodological flaws of quasi-experimental medical informatics studies, which have the potential to introduce bias, were also identified. In addition, a summary table outlining a relative hierarchy and nomenclature of quasi-experimental study designs is described. In general, the higher the design is in the hierarchy, the greater the internal validity that the study traditionally possesses because the evidence of the potential causation between the intervention and the outcome is strengthened. 4

We then performed a systematic review of four years of publications from two informatics journals. First, we determined the number of quasi-experimental studies. We then classified these studies on the above-mentioned hierarchy. We also classified the quasi-experimental studies according to their application domain. The categories of application domains employed were based on categorization used by Yearbooks of Medical Informatics 1992–2005 and were similar to the categories of application domains employed by Annual Symposiums of the American Medical Informatics Association. 11 The categories were (1) health and clinical management; (2) patient records; (3) health information systems; (4) medical signal processing and biomedical imaging; (5) decision support, knowledge representation, and management; (6) education and consumer informatics; and (7) bioinformatics. Because the quasi-experimental study design has recognized limitations, we sought to determine whether authors acknowledged the potential limitations of this design. Examples of acknowledgment included mention of lack of randomization, the potential for regression to the mean, the presence of temporal confounders and the mention of another design that would have more internal validity.

All original scientific manuscripts published between January 2000 and December 2003 in the Journal of the American Medical Informatics Association (JAMIA) and the International Journal of Medical Informatics (IJMI) were reviewed. One author (ADH) reviewed all the papers to identify the number of quasi-experimental studies. Other authors (ADH, JCM, JF) then independently reviewed all the studies identified as quasi-experimental. The three authors then convened as a group to resolve any disagreements in study classification, application domain, and acknowledgment of limitations.

Results and Discussion

What is a quasi-experiment.

Quasi-experiments are studies that aim to evaluate interventions but that do not use randomization. Similar to randomized trials, quasi-experiments aim to demonstrate causality between an intervention and an outcome. Quasi-experimental studies can use both preintervention and postintervention measurements as well as nonrandomly selected control groups.

Using this basic definition, it is evident that many published studies in medical informatics utilize the quasi-experimental design. Although the randomized controlled trial is generally considered to have the highest level of credibility with regard to assessing causality, in medical informatics, researchers often choose not to randomize the intervention for one or more reasons: (1) ethical considerations, (2) difficulty of randomizing subjects, (3) difficulty to randomize by locations (e.g., by wards), (4) small available sample size. Each of these reasons is discussed below.

Ethical considerations typically will not allow random withholding of an intervention with known efficacy. Thus, if the efficacy of an intervention has not been established, a randomized controlled trial is the design of choice to determine efficacy. But if the intervention under study incorporates an accepted, well-established therapeutic intervention, or if the intervention has either questionable efficacy or safety based on previously conducted studies, then the ethical issues of randomizing patients are sometimes raised. In the area of medical informatics, it is often believed prior to an implementation that an informatics intervention will likely be beneficial and thus medical informaticians and hospital administrators are often reluctant to randomize medical informatics interventions. In addition, there is often pressure to implement the intervention quickly because of its believed efficacy, thus not allowing researchers sufficient time to plan a randomized trial.

For medical informatics interventions, it is often difficult to randomize the intervention to individual patients or to individual informatics users. So while this randomization is technically possible, it is underused and thus compromises the eventual strength of concluding that an informatics intervention resulted in an outcome. For example, randomly allowing only half of medical residents to use pharmacy order-entry software at a tertiary care hospital is a scenario that hospital administrators and informatics users may not agree to for numerous reasons.

Similarly, informatics interventions often cannot be randomized to individual locations. Using the pharmacy order-entry system example, it may be difficult to randomize use of the system to only certain locations in a hospital or portions of certain locations. For example, if the pharmacy order-entry system involves an educational component, then people may apply the knowledge learned to nonintervention wards, thereby potentially masking the true effect of the intervention. When a design using randomized locations is employed successfully, the locations may be different in other respects (confounding variables), and this further complicates the analysis and interpretation.

In situations where it is known that only a small sample size will be available to test the efficacy of an intervention, randomization may not be a viable option. Randomization is beneficial because on average it tends to evenly distribute both known and unknown confounding variables between the intervention and control group. However, when the sample size is small, randomization may not adequately accomplish this balance. Thus, alternative design and analytical methods are often used in place of randomization when only small sample sizes are available.

What Are the Threats to Establishing Causality When Using Quasi-experimental Designs in Medical Informatics?

The lack of random assignment is the major weakness of the quasi-experimental study design. Associations identified in quasi-experiments meet one important requirement of causality since the intervention precedes the measurement of the outcome. Another requirement is that the outcome can be demonstrated to vary statistically with the intervention. Unfortunately, statistical association does not imply causality, especially if the study is poorly designed. Thus, in many quasi-experiments, one is most often left with the question: “Are there alternative explanations for the apparent causal association?” If these alternative explanations are credible, then the evidence of causation is less convincing. These rival hypotheses, or alternative explanations, arise from principles of epidemiologic study design.

Shadish et al. 4 outline nine threats to internal validity that are outlined in ▶ . Internal validity is defined as the degree to which observed changes in outcomes can be correctly inferred to be caused by an exposure or an intervention. In quasi-experimental studies of medical informatics, we believe that the methodological principles that most often result in alternative explanations for the apparent causal effect include (a) difficulty in measuring or controlling for important confounding variables, particularly unmeasured confounding variables, which can be viewed as a subset of the selection threat in ▶ ; (b) results being explained by the statistical principle of regression to the mean . Each of these latter two principles is discussed in turn.

Threats to Internal Validity

Adapted from Shadish et al. 4

An inability to sufficiently control for important confounding variables arises from the lack of randomization. A variable is a confounding variable if it is associated with the exposure of interest and is also associated with the outcome of interest; the confounding variable leads to a situation where a causal association between a given exposure and an outcome is observed as a result of the influence of the confounding variable. For example, in a study aiming to demonstrate that the introduction of a pharmacy order-entry system led to lower pharmacy costs, there are a number of important potential confounding variables (e.g., severity of illness of the patients, knowledge and experience of the software users, other changes in hospital policy) that may have differed in the preintervention and postintervention time periods ( ▶ ). In a multivariable regression, the first confounding variable could be addressed with severity of illness measures, but the second confounding variable would be difficult if not nearly impossible to measure and control. In addition, potential confounding variables that are unmeasured or immeasurable cannot be controlled for in nonrandomized quasi-experimental study designs and can only be properly controlled by the randomization process in randomized controlled trials.

Example of confounding. To get the true effect of the intervention of interest, we need to control for the confounding variable.

Another important threat to establishing causality is regression to the mean. 12 , 13 , 14 This widespread statistical phenomenon can result in wrongly concluding that an effect is due to the intervention when in reality it is due to chance. The phenomenon was first described in 1886 by Francis Galton who measured the adult height of children and their parents. He noted that when the average height of the parents was greater than the mean of the population, the children tended to be shorter than their parents, and conversely, when the average height of the parents was shorter than the population mean, the children tended to be taller than their parents.

In medical informatics, what often triggers the development and implementation of an intervention is a rise in the rate above the mean or norm. For example, increasing pharmacy costs and adverse events may prompt hospital informatics personnel to design and implement pharmacy order-entry systems. If this rise in costs or adverse events is really just an extreme observation that is still within the normal range of the hospital's pharmaceutical costs (i.e., the mean pharmaceutical cost for the hospital has not shifted), then the statistical principle of regression to the mean predicts that these elevated rates will tend to decline even without intervention. However, often informatics personnel and hospital administrators cannot wait passively for this decline to occur. Therefore, hospital personnel often implement one or more interventions, and if a decline in the rate occurs, they may mistakenly conclude that the decline is causally related to the intervention. In fact, an alternative explanation for the finding could be regression to the mean.

What Are the Different Quasi-experimental Study Designs?

In the social sciences literature, quasi-experimental studies are divided into four study design groups 4 , 6 :

• Quasi-experimental designs without control groups
• Quasi-experimental designs that use control groups but no pretest
• Quasi-experimental designs that use control groups and pretests
• Interrupted time-series designs

There is a relative hierarchy within these categories of study designs, with category D studies being sounder than categories C, B, or A in terms of establishing causality. Thus, if feasible from a design and implementation point of view, investigators should aim to design studies that fall in to the higher rated categories. Shadish et al. 4 discuss 17 possible designs, with seven designs falling into category A, three designs in category B, and six designs in category C, and one major design in category D. In our review, we determined that most medical informatics quasi-experiments could be characterized by 11 of 17 designs, with six study designs in category A, one in category B, three designs in category C, and one design in category D because the other study designs were not used or feasible in the medical informatics literature. Thus, for simplicity, we have summarized the 11 study designs most relevant to medical informatics research in ▶ .

Relative Hierarchy of Quasi-experimental Designs

O = Observational Measurement; X = Intervention Under Study. Time moves from left to right.

The nomenclature and relative hierarchy were used in the systematic review of four years of JAMIA and the IJMI. Similar to the relative hierarchy that exists in the evidence-based literature that assigns a hierarchy to randomized controlled trials, cohort studies, case-control studies, and case series, the hierarchy in ▶ is not absolute in that in some cases, it may be infeasible to perform a higher level study. For example, there may be instances where an A6 design established stronger causality than a B1 design. 15 , 16 , 17

Quasi-experimental Designs without Control Groups

Here, X is the intervention and O is the outcome variable (this notation is continued throughout the article). In this study design, an intervention (X) is implemented and a posttest observation (O1) is taken. For example, X could be the introduction of a pharmacy order-entry intervention and O1 could be the pharmacy costs following the intervention. This design is the weakest of the quasi-experimental designs that are discussed in this article. Without any pretest observations or a control group, there are multiple threats to internal validity. Unfortunately, this study design is often used in medical informatics when new software is introduced since it may be difficult to have pretest measurements due to time, technical, or cost constraints.

This is a commonly used study design. A single pretest measurement is taken (O1), an intervention (X) is implemented, and a posttest measurement is taken (O2). In this instance, period O1 frequently serves as the “control” period. For example, O1 could be pharmacy costs prior to the intervention, X could be the introduction of a pharmacy order-entry system, and O2 could be the pharmacy costs following the intervention. Including a pretest provides some information about what the pharmacy costs would have been had the intervention not occurred.

The advantage of this study design over A2 is that adding a second pretest prior to the intervention helps provide evidence that can be used to refute the phenomenon of regression to the mean and confounding as alternative explanations for any observed association between the intervention and the posttest outcome. For example, in a study where a pharmacy order-entry system led to lower pharmacy costs (O3 < O2 and O1), if one had two preintervention measurements of pharmacy costs (O1 and O2) and they were both elevated, this would suggest that there was a decreased likelihood that O3 is lower due to confounding and regression to the mean. Similarly, extending this study design by increasing the number of measurements postintervention could also help to provide evidence against confounding and regression to the mean as alternate explanations for observed associations.

This design involves the inclusion of a nonequivalent dependent variable ( b ) in addition to the primary dependent variable ( a ). Variables a and b should assess similar constructs; that is, the two measures should be affected by similar factors and confounding variables except for the effect of the intervention. Variable a is expected to change because of the intervention X, whereas variable b is not. Taking our example, variable a could be pharmacy costs and variable b could be the length of stay of patients. If our informatics intervention is aimed at decreasing pharmacy costs, we would expect to observe a decrease in pharmacy costs but not in the average length of stay of patients. However, a number of important confounding variables, such as severity of illness and knowledge of software users, might affect both outcome measures. Thus, if the average length of stay did not change following the intervention but pharmacy costs did, then the data are more convincing than if just pharmacy costs were measured.

The Removed-Treatment Design

This design adds a third posttest measurement (O3) to the one-group pretest-posttest design and then removes the intervention before a final measure (O4) is made. The advantage of this design is that it allows one to test hypotheses about the outcome in the presence of the intervention and in the absence of the intervention. Thus, if one predicts a decrease in the outcome between O1 and O2 (after implementation of the intervention), then one would predict an increase in the outcome between O3 and O4 (after removal of the intervention). One caveat is that if the intervention is thought to have persistent effects, then O4 needs to be measured after these effects are likely to have disappeared. For example, a study would be more convincing if it demonstrated that pharmacy costs decreased after pharmacy order-entry system introduction (O2 and O3 less than O1) and that when the order-entry system was removed or disabled, the costs increased (O4 greater than O2 and O3 and closer to O1). In addition, there are often ethical issues in this design in terms of removing an intervention that may be providing benefit.

The Repeated-Treatment Design

The advantage of this design is that it demonstrates reproducibility of the association between the intervention and the outcome. For example, the association is more likely to be causal if one demonstrates that a pharmacy order-entry system results in decreased pharmacy costs when it is first introduced and again when it is reintroduced following an interruption of the intervention. As for design A5, the assumption must be made that the effect of the intervention is transient, which is most often applicable to medical informatics interventions. Because in this design, subjects may serve as their own controls, this may yield greater statistical efficiency with fewer numbers of subjects.

Quasi-experimental Designs That Use a Control Group but No Pretest

An intervention X is implemented for one group and compared to a second group. The use of a comparison group helps prevent certain threats to validity including the ability to statistically adjust for confounding variables. Because in this study design, the two groups may not be equivalent (assignment to the groups is not by randomization), confounding may exist. For example, suppose that a pharmacy order-entry intervention was instituted in the medical intensive care unit (MICU) and not the surgical intensive care unit (SICU). O1 would be pharmacy costs in the MICU after the intervention and O2 would be pharmacy costs in the SICU after the intervention. The absence of a pretest makes it difficult to know whether a change has occurred in the MICU. Also, the absence of pretest measurements comparing the SICU to the MICU makes it difficult to know whether differences in O1 and O2 are due to the intervention or due to other differences in the two units (confounding variables).

Quasi-experimental Designs That Use Control Groups and Pretests

The reader should note that with all the studies in this category, the intervention is not randomized. The control groups chosen are comparison groups. Obtaining pretest measurements on both the intervention and control groups allows one to assess the initial comparability of the groups. The assumption is that if the intervention and the control groups are similar at the pretest, the smaller the likelihood there is of important confounding variables differing between the two groups.

The use of both a pretest and a comparison group makes it easier to avoid certain threats to validity. However, because the two groups are nonequivalent (assignment to the groups is not by randomization), selection bias may exist. Selection bias exists when selection results in differences in unit characteristics between conditions that may be related to outcome differences. For example, suppose that a pharmacy order-entry intervention was instituted in the MICU and not the SICU. If preintervention pharmacy costs in the MICU (O1a) and SICU (O1b) are similar, it suggests that it is less likely that there are differences in the important confounding variables between the two units. If MICU postintervention costs (O2a) are less than preintervention MICU costs (O1a), but SICU costs (O1b) and (O2b) are similar, this suggests that the observed outcome may be causally related to the intervention.

In this design, the pretests are administered at two different times. The main advantage of this design is that it controls for potentially different time-varying confounding effects in the intervention group and the comparison group. In our example, measuring points O1 and O2 would allow for the assessment of time-dependent changes in pharmacy costs, e.g., due to differences in experience of residents, preintervention between the intervention and control group, and whether these changes were similar or different.

With this study design, the researcher administers an intervention at a later time to a group that initially served as a nonintervention control. The advantage of this design over design C2 is that it demonstrates reproducibility in two different settings. This study design is not limited to two groups; in fact, the study results have greater validity if the intervention effect is replicated in different groups at multiple times. In the example of a pharmacy order-entry system, one could implement or intervene in the MICU and then at a later time, intervene in the SICU. This latter design is often very applicable to medical informatics where new technology and new software is often introduced or made available gradually.

Interrupted Time-Series Designs

An interrupted time-series design is one in which a string of consecutive observations equally spaced in time is interrupted by the imposition of a treatment or intervention. The advantage of this design is that with multiple measurements both pre- and postintervention, it is easier to address and control for confounding and regression to the mean. In addition, statistically, there is a more robust analytic capability, and there is the ability to detect changes in the slope or intercept as a result of the intervention in addition to a change in the mean values. 18 A change in intercept could represent an immediate effect while a change in slope could represent a gradual effect of the intervention on the outcome. In the example of a pharmacy order-entry system, O1 through O5 could represent monthly pharmacy costs preintervention and O6 through O10 monthly pharmacy costs post the introduction of the pharmacy order-entry system. Interrupted time-series designs also can be further strengthened by incorporating many of the design features previously mentioned in other categories (such as removal of the treatment, inclusion of a nondependent outcome variable, or the addition of a control group).

Systematic Review Results

The results of the systematic review are in ▶ . In the four-year period of JAMIA publications that the authors reviewed, 25 quasi-experimental studies among 22 articles were published. Of these 25, 15 studies were of category A, five studies were of category B, two studies were of category C, and no studies were of category D. Although there were no studies of category D (interrupted time-series analyses), three of the studies classified as category A had data collected that could have been analyzed as an interrupted time-series analysis. Nine of the 25 studies (36%) mentioned at least one of the potential limitations of the quasi-experimental study design. In the four-year period of IJMI publications reviewed by the authors, nine quasi-experimental studies among eight manuscripts were published. Of these nine, five studies were of category A, one of category B, one of category C, and two of category D. Two of the nine studies (22%) mentioned at least one of the potential limitations of the quasi-experimental study design.

Systematic Review of Four Years of Quasi-designs in JAMIA

JAMIA = Journal of the American Medical Informatics Association; IJMI = International Journal of Medical Informatics.

In addition, three studies from JAMIA were based on a counterbalanced design. A counterbalanced design is a higher order study design than other studies in category A. The counterbalanced design is sometimes referred to as a Latin-square arrangement. In this design, all subjects receive all the different interventions but the order of intervention assignment is not random. 19 This design can only be used when the intervention is compared against some existing standard, for example, if a new PDA-based order entry system is to be compared to a computer terminal–based order entry system. In this design, all subjects receive the new PDA-based order entry system and the old computer terminal-based order entry system. The counterbalanced design is a within-participants design, where the order of the intervention is varied (e.g., one group is given software A followed by software B and another group is given software B followed by software A). The counterbalanced design is typically used when the available sample size is small, thus preventing the use of randomization. This design also allows investigators to study the potential effect of ordering of the informatics intervention.

Although quasi-experimental study designs are ubiquitous in the medical informatics literature, as evidenced by 34 studies in the past four years of the two informatics journals, little has been written about the benefits and limitations of the quasi-experimental approach. As we have outlined in this paper, a relative hierarchy and nomenclature of quasi-experimental study designs exist, with some designs being more likely than others to permit causal interpretations of observed associations. Strengths and limitations of a particular study design should be discussed when presenting data collected in the setting of a quasi-experimental study. Future medical informatics investigators should choose the strongest design that is feasible given the particular circumstances.

Supplementary Material

Dr. Harris was supported by NIH grants K23 AI01752-01A1 and R01 AI60859-01A1. Dr. Perencevich was supported by a VA Health Services Research and Development Service (HSR&D) Research Career Development Award (RCD-02026-1). Dr. Finkelstein was supported by NIH grant RO1 HL71690.

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• Guide to Experimental Design | Overview, Steps, & Examples

Guide to Experimental Design | Overview, 5 steps & Examples

Published on December 3, 2019 by Rebecca Bevans . Revised on June 21, 2023.

Experiments are used to study causal relationships . You manipulate one or more independent variables and measure their effect on one or more dependent variables.

Experimental design create a set of procedures to systematically test a hypothesis . A good experimental design requires a strong understanding of the system you are studying.

There are five key steps in designing an experiment:

• Consider your variables and how they are related
• Write a specific, testable hypothesis
• Design experimental treatments to manipulate your independent variable
• Assign subjects to groups, either between-subjects or within-subjects
• Plan how you will measure your dependent variable

For valid conclusions, you also need to select a representative sample and control any  extraneous variables that might influence your results. If random assignment of participants to control and treatment groups is impossible, unethical, or highly difficult, consider an observational study instead. This minimizes several types of research bias, particularly sampling bias , survivorship bias , and attrition bias as time passes.

Table of contents

Step 1: define your variables, step 2: write your hypothesis, step 3: design your experimental treatments, step 4: assign your subjects to treatment groups, step 5: measure your dependent variable, other interesting articles, frequently asked questions about experiments.

You should begin with a specific research question . We will work with two research question examples, one from health sciences and one from ecology:

To translate your research question into an experimental hypothesis, you need to define the main variables and make predictions about how they are related.

Start by simply listing the independent and dependent variables .

Then you need to think about possible extraneous and confounding variables and consider how you might control  them in your experiment.

Finally, you can put these variables together into a diagram. Use arrows to show the possible relationships between variables and include signs to show the expected direction of the relationships.

Here we predict that increasing temperature will increase soil respiration and decrease soil moisture, while decreasing soil moisture will lead to decreased soil respiration.

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Now that you have a strong conceptual understanding of the system you are studying, you should be able to write a specific, testable hypothesis that addresses your research question.

The next steps will describe how to design a controlled experiment . In a controlled experiment, you must be able to:

• Systematically and precisely manipulate the independent variable(s).
• Precisely measure the dependent variable(s).
• Control any potential confounding variables.

If your study system doesn’t match these criteria, there are other types of research you can use to answer your research question.

How you manipulate the independent variable can affect the experiment’s external validity – that is, the extent to which the results can be generalized and applied to the broader world.

First, you may need to decide how widely to vary your independent variable.

• just slightly above the natural range for your study region.
• over a wider range of temperatures to mimic future warming.
• over an extreme range that is beyond any possible natural variation.

Second, you may need to choose how finely to vary your independent variable. Sometimes this choice is made for you by your experimental system, but often you will need to decide, and this will affect how much you can infer from your results.

• a categorical variable : either as binary (yes/no) or as levels of a factor (no phone use, low phone use, high phone use).
• a continuous variable (minutes of phone use measured every night).

How you apply your experimental treatments to your test subjects is crucial for obtaining valid and reliable results.

First, you need to consider the study size : how many individuals will be included in the experiment? In general, the more subjects you include, the greater your experiment’s statistical power , which determines how much confidence you can have in your results.

Then you need to randomly assign your subjects to treatment groups . Each group receives a different level of the treatment (e.g. no phone use, low phone use, high phone use).

You should also include a control group , which receives no treatment. The control group tells us what would have happened to your test subjects without any experimental intervention.

When assigning your subjects to groups, there are two main choices you need to make:

• A completely randomized design vs a randomized block design .
• A between-subjects design vs a within-subjects design .

Randomization

An experiment can be completely randomized or randomized within blocks (aka strata):

• In a completely randomized design , every subject is assigned to a treatment group at random.
• In a randomized block design (aka stratified random design), subjects are first grouped according to a characteristic they share, and then randomly assigned to treatments within those groups.

Sometimes randomization isn’t practical or ethical , so researchers create partially-random or even non-random designs. An experimental design where treatments aren’t randomly assigned is called a quasi-experimental design .

Between-subjects vs. within-subjects

In a between-subjects design (also known as an independent measures design or classic ANOVA design), individuals receive only one of the possible levels of an experimental treatment.

In medical or social research, you might also use matched pairs within your between-subjects design to make sure that each treatment group contains the same variety of test subjects in the same proportions.

In a within-subjects design (also known as a repeated measures design), every individual receives each of the experimental treatments consecutively, and their responses to each treatment are measured.

Within-subjects or repeated measures can also refer to an experimental design where an effect emerges over time, and individual responses are measured over time in order to measure this effect as it emerges.

Counterbalancing (randomizing or reversing the order of treatments among subjects) is often used in within-subjects designs to ensure that the order of treatment application doesn’t influence the results of the experiment.

Finally, you need to decide how you’ll collect data on your dependent variable outcomes. You should aim for reliable and valid measurements that minimize research bias or error.

Some variables, like temperature, can be objectively measured with scientific instruments. Others may need to be operationalized to turn them into measurable observations.

• Ask participants to record what time they go to sleep and get up each day.
• Ask participants to wear a sleep tracker.

How precisely you measure your dependent variable also affects the kinds of statistical analysis you can use on your data.

Experiments are always context-dependent, and a good experimental design will take into account all of the unique considerations of your study system to produce information that is both valid and relevant to your research question.

If you want to know more about statistics , methodology , or research bias , make sure to check out some of our other articles with explanations and examples.

• Student’s  t -distribution
• Normal distribution
• Null and Alternative Hypotheses
• Chi square tests
• Confidence interval
• Cluster sampling
• Stratified sampling
• Data cleansing
• Reproducibility vs Replicability
• Peer review
• Likert scale

Research bias

• Implicit bias
• Framing effect
• Cognitive bias
• Placebo effect
• Hawthorne effect
• Hindsight bias
• Affect heuristic

Experimental design means planning a set of procedures to investigate a relationship between variables . To design a controlled experiment, you need:

• A testable hypothesis
• At least one independent variable that can be precisely manipulated
• At least one dependent variable that can be precisely measured

When designing the experiment, you decide:

• How you will manipulate the variable(s)
• How you will control for any potential confounding variables
• How many subjects or samples will be included in the study
• How subjects will be assigned to treatment levels

Experimental design is essential to the internal and external validity of your experiment.

The key difference between observational studies and experimental designs is that a well-done observational study does not influence the responses of participants, while experiments do have some sort of treatment condition applied to at least some participants by random assignment .

A confounding variable , also called a confounder or confounding factor, is a third variable in a study examining a potential cause-and-effect relationship.

A confounding variable is related to both the supposed cause and the supposed effect of the study. It can be difficult to separate the true effect of the independent variable from the effect of the confounding variable.

In your research design , it’s important to identify potential confounding variables and plan how you will reduce their impact.

In a between-subjects design , every participant experiences only one condition, and researchers assess group differences between participants in various conditions.

In a within-subjects design , each participant experiences all conditions, and researchers test the same participants repeatedly for differences between conditions.

The word “between” means that you’re comparing different conditions between groups, while the word “within” means you’re comparing different conditions within the same group.

An experimental group, also known as a treatment group, receives the treatment whose effect researchers wish to study, whereas a control group does not. They should be identical in all other ways.

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Quasi-Experiment: Understand What It Is, Types & Examples

Discover the concept of quasi-experiment, its various types, real-world examples, and how QuestionPro aids in conducting these studies.

Quasi-experimental research designs have gained significant recognition in the scientific community due to their unique ability to study cause-and-effect relationships in real-world settings. Unlike true experiments, quasi-experiment lack random assignment of participants to groups, making them more practical and ethical in certain situations. In this article, we will delve into the concept, applications, and advantages of quasi-experiments, shedding light on their relevance and significance in the scientific realm.

What Is A Quasi-Experiment Research Design?

Quasi-experimental research designs are research methodologies that resemble true experiments but lack the randomized assignment of participants to groups. In a true experiment, researchers randomly assign participants to either an experimental group or a control group, allowing for a comparison of the effects of an independent variable on the dependent variable. However, in quasi-experiments, this random assignment is often not possible or ethically permissible, leading to the adoption of alternative strategies.

Types Of Quasi-Experimental Designs

There are several types of quasi-experiment designs to study causal relationships in specific contexts. Some common types include:

Non-Equivalent Groups Design

This design involves selecting pre-existing groups that differ in some key characteristics and comparing their responses to the independent variable. Although the researcher does not randomly assign the groups, they can still examine the effects of the independent variable.

Regression Discontinuity

This design utilizes a cutoff point or threshold to determine which participants receive the treatment or intervention. It assumes that participants on either side of the cutoff are similar in all other aspects, except for their exposure to the independent variable.

Interrupted Time Series Design

This design involves measuring the dependent variable multiple times before and after the introduction of an intervention or treatment. By comparing the trends in the dependent variable, researchers can infer the impact of the intervention.

Natural Experiments

Natural experiments take advantage of naturally occurring events or circumstances that mimic the random assignment found in true experiments. Participants are exposed to different conditions in situations identified by researchers without any manipulation from them.

Application of the Quasi-Experiment Design

Quasi-experimental research designs find applications in various fields, ranging from education to public health and beyond. One significant advantage of quasi-experiments is their feasibility in real-world settings where randomization is not always possible or ethical.

Ethical Reasons

Ethical concerns often arise in research when randomizing participants to different groups could potentially deny individuals access to beneficial treatments or interventions. In such cases, quasi-experimental designs provide an ethical alternative, allowing researchers to study the impact of interventions without depriving anyone of potential benefits.

Examples Of Quasi-Experimental Design

Let’s explore a few examples of quasi-experimental designs to understand their application in different contexts.

Design Of Non-Equivalent Groups

Determining the effectiveness of math apps in supplementing math classes.

Imagine a study aiming to determine the effectiveness of math apps in supplementing traditional math classes in a school. Randomly assigning students to different groups might be impractical or disrupt the existing classroom structure. Instead, researchers can select two comparable classes, one receiving the math app intervention and the other continuing with traditional teaching methods. By comparing the performance of the two groups, researchers can draw conclusions about the app’s effectiveness.

To conduct a quasi-experiment study like the one mentioned above, researchers can utilize QuestionPro , an advanced research platform that offers comprehensive survey and data analysis tools. With QuestionPro, researchers can design surveys to collect data, analyze results, and gain valuable insights for their quasi-experimental research.

How QuestionPro Helps In Quasi-Experimental Research?

QuestionPro’s powerful features, such as random assignment of participants, survey branching, and data visualization, enable researchers to efficiently conduct and analyze quasi-experimental studies. The platform provides a user-friendly interface and robust reporting capabilities, empowering researchers to gather data, explore relationships, and draw meaningful conclusions.

In some cases, researchers can leverage natural experiments to examine causal relationships. 

Determining The Effectiveness Of Teaching Modern Leadership Techniques In Start-Up Businesses

Consider a study evaluating the effectiveness of teaching modern leadership techniques in start-up businesses. Instead of artificially assigning businesses to different groups, researchers can observe those that naturally adopt modern leadership techniques and compare their outcomes to those of businesses that have not implemented such practices.

Advantages and Disadvantages Of The Quasi-Experimental Design

Quasi-experimental designs offer several advantages over true experiments, making them valuable tools in research:

• Scope of the research : Quasi-experiments allow researchers to study cause-and-effect relationships in real-world settings, providing valuable insights into complex phenomena that may be challenging to replicate in a controlled laboratory environment.
• Regression Discontinuity : Researchers can utilize regression discontinuity to evaluate the effects of interventions or treatments when random assignment is not feasible. This design leverages existing data and naturally occurring thresholds to draw causal inferences.

Disadvantage

Lack of random assignment : Quasi-experimental designs lack the random assignment of participants, which introduces the possibility of confounding variables affecting the results. Researchers must carefully consider potential alternative explanations for observed effects.

What Are The Different Quasi-Experimental Study Designs?

Quasi-experimental designs encompass various approaches, including nonequivalent group designs, interrupted time series designs, and natural experiments. Each design offers unique advantages and limitations, providing researchers with versatile tools to explore causal relationships in different contexts.

Example Of The Natural Experiment Approach

Researchers interested in studying the impact of a public health campaign aimed at reducing smoking rates may take advantage of a natural experiment. By comparing smoking rates in a region that has implemented the campaign to a similar region that has not, researchers can examine the effectiveness of the intervention.

Differences Between Quasi-Experiments And True Experiments

Quasi-experiments and true experiments differ primarily in their ability to randomly assign participants to groups. While true experiments provide a higher level of control, quasi-experiments offer practical and ethical alternatives in situations where randomization is not feasible or desirable.

Example Comparing A True Experiment And Quasi-Experiment

In a true experiment investigating the effects of a new medication on a specific condition, researchers would randomly assign participants to either the experimental group, which receives the medication, or the control group, which receives a placebo. In a quasi-experiment, researchers might instead compare patients who voluntarily choose to take the medication to those who do not, examining the differences in outcomes between the two groups.

Quasi-Experiment: A Quick Wrap-Up

Quasi-experimental research designs play a vital role in scientific inquiry by allowing researchers to investigate cause-and-effect relationships in real-world settings. These designs offer practical and ethical alternatives to true experiments, making them valuable tools in various fields of study. With their versatility and applicability, quasi-experimental designs continue to contribute to our understanding of complex phenomena.

Turn Your Data Into Easy-To-Understand And Dynamic Stories

When you wish to explain any complex data, it’s always advised to break it down into simpler visuals or stories. This is where Mind the Graph comes in. It is a platform that helps researchers and scientists to turn their data into easy-to-understand and dynamic stories, helping the audience understand the concepts better. Sign Up now to explore the library of scientific infographics . 

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Chapter 7: Nonexperimental Research

Quasi-Experimental Research

Learning Objectives

• Explain what quasi-experimental research is and distinguish it clearly from both experimental and correlational research.
• Describe three different types of quasi-experimental research designs (nonequivalent groups, pretest-posttest, and interrupted time series) and identify examples of each one.

The prefix  quasi  means “resembling.” Thus quasi-experimental research is research that resembles experimental research but is not true experimental research. Although the independent variable is manipulated, participants are not randomly assigned to conditions or orders of conditions (Cook & Campbell, 1979). [1] Because the independent variable is manipulated before the dependent variable is measured, quasi-experimental research eliminates the directionality problem. But because participants are not randomly assigned—making it likely that there are other differences between conditions—quasi-experimental research does not eliminate the problem of confounding variables. In terms of internal validity, therefore, quasi-experiments are generally somewhere between correlational studies and true experiments.

Quasi-experiments are most likely to be conducted in field settings in which random assignment is difficult or impossible. They are often conducted to evaluate the effectiveness of a treatment—perhaps a type of psychotherapy or an educational intervention. There are many different kinds of quasi-experiments, but we will discuss just a few of the most common ones here.

Nonequivalent Groups Design

Recall that when participants in a between-subjects experiment are randomly assigned to conditions, the resulting groups are likely to be quite similar. In fact, researchers consider them to be equivalent. When participants are not randomly assigned to conditions, however, the resulting groups are likely to be dissimilar in some ways. For this reason, researchers consider them to be nonequivalent. A  nonequivalent groups design , then, is a between-subjects design in which participants have not been randomly assigned to conditions.

Imagine, for example, a researcher who wants to evaluate a new method of teaching fractions to third graders. One way would be to conduct a study with a treatment group consisting of one class of third-grade students and a control group consisting of another class of third-grade students. This design would be a nonequivalent groups design because the students are not randomly assigned to classes by the researcher, which means there could be important differences between them. For example, the parents of higher achieving or more motivated students might have been more likely to request that their children be assigned to Ms. Williams’s class. Or the principal might have assigned the “troublemakers” to Mr. Jones’s class because he is a stronger disciplinarian. Of course, the teachers’ styles, and even the classroom environments, might be very different and might cause different levels of achievement or motivation among the students. If at the end of the study there was a difference in the two classes’ knowledge of fractions, it might have been caused by the difference between the teaching methods—but it might have been caused by any of these confounding variables.

Of course, researchers using a nonequivalent groups design can take steps to ensure that their groups are as similar as possible. In the present example, the researcher could try to select two classes at the same school, where the students in the two classes have similar scores on a standardized math test and the teachers are the same sex, are close in age, and have similar teaching styles. Taking such steps would increase the internal validity of the study because it would eliminate some of the most important confounding variables. But without true random assignment of the students to conditions, there remains the possibility of other important confounding variables that the researcher was not able to control.

Pretest-Posttest Design

In a  pretest-posttest design , the dependent variable is measured once before the treatment is implemented and once after it is implemented. Imagine, for example, a researcher who is interested in the effectiveness of an antidrug education program on elementary school students’ attitudes toward illegal drugs. The researcher could measure the attitudes of students at a particular elementary school during one week, implement the antidrug program during the next week, and finally, measure their attitudes again the following week. The pretest-posttest design is much like a within-subjects experiment in which each participant is tested first under the control condition and then under the treatment condition. It is unlike a within-subjects experiment, however, in that the order of conditions is not counterbalanced because it typically is not possible for a participant to be tested in the treatment condition first and then in an “untreated” control condition.

If the average posttest score is better than the average pretest score, then it makes sense to conclude that the treatment might be responsible for the improvement. Unfortunately, one often cannot conclude this with a high degree of certainty because there may be other explanations for why the posttest scores are better. One category of alternative explanations goes under the name of  history . Other things might have happened between the pretest and the posttest. Perhaps an antidrug program aired on television and many of the students watched it, or perhaps a celebrity died of a drug overdose and many of the students heard about it. Another category of alternative explanations goes under the name of  maturation . Participants might have changed between the pretest and the posttest in ways that they were going to anyway because they are growing and learning. If it were a yearlong program, participants might become less impulsive or better reasoners and this might be responsible for the change.

Another alternative explanation for a change in the dependent variable in a pretest-posttest design is  regression to the mean . This refers to the statistical fact that an individual who scores extremely on a variable on one occasion will tend to score less extremely on the next occasion. For example, a bowler with a long-term average of 150 who suddenly bowls a 220 will almost certainly score lower in the next game. Her score will “regress” toward her mean score of 150. Regression to the mean can be a problem when participants are selected for further study  because  of their extreme scores. Imagine, for example, that only students who scored especially low on a test of fractions are given a special training program and then retested. Regression to the mean all but guarantees that their scores will be higher even if the training program has no effect. A closely related concept—and an extremely important one in psychological research—is  spontaneous remission . This is the tendency for many medical and psychological problems to improve over time without any form of treatment. The common cold is a good example. If one were to measure symptom severity in 100 common cold sufferers today, give them a bowl of chicken soup every day, and then measure their symptom severity again in a week, they would probably be much improved. This does not mean that the chicken soup was responsible for the improvement, however, because they would have been much improved without any treatment at all. The same is true of many psychological problems. A group of severely depressed people today is likely to be less depressed on average in 6 months. In reviewing the results of several studies of treatments for depression, researchers Michael Posternak and Ivan Miller found that participants in waitlist control conditions improved an average of 10 to 15% before they received any treatment at all (Posternak & Miller, 2001) [2] . Thus one must generally be very cautious about inferring causality from pretest-posttest designs.

Does Psychotherapy Work?

Early studies on the effectiveness of psychotherapy tended to use pretest-posttest designs. In a classic 1952 article, researcher Hans Eysenck summarized the results of 24 such studies showing that about two thirds of patients improved between the pretest and the posttest (Eysenck, 1952) [3] . But Eysenck also compared these results with archival data from state hospital and insurance company records showing that similar patients recovered at about the same rate  without  receiving psychotherapy. This parallel suggested to Eysenck that the improvement that patients showed in the pretest-posttest studies might be no more than spontaneous remission. Note that Eysenck did not conclude that psychotherapy was ineffective. He merely concluded that there was no evidence that it was, and he wrote of “the necessity of properly planned and executed experimental studies into this important field” (p. 323). You can read the entire article here: Classics in the History of Psychology .

Fortunately, many other researchers took up Eysenck’s challenge, and by 1980 hundreds of experiments had been conducted in which participants were randomly assigned to treatment and control conditions, and the results were summarized in a classic book by Mary Lee Smith, Gene Glass, and Thomas Miller (Smith, Glass, & Miller, 1980) [4] . They found that overall psychotherapy was quite effective, with about 80% of treatment participants improving more than the average control participant. Subsequent research has focused more on the conditions under which different types of psychotherapy are more or less effective.

Interrupted Time Series Design

A variant of the pretest-posttest design is the  interrupted time-series design . A time series is a set of measurements taken at intervals over a period of time. For example, a manufacturing company might measure its workers’ productivity each week for a year. In an interrupted time series-design, a time series like this one is “interrupted” by a treatment. In one classic example, the treatment was the reduction of the work shifts in a factory from 10 hours to 8 hours (Cook & Campbell, 1979) [5] . Because productivity increased rather quickly after the shortening of the work shifts, and because it remained elevated for many months afterward, the researcher concluded that the shortening of the shifts caused the increase in productivity. Notice that the interrupted time-series design is like a pretest-posttest design in that it includes measurements of the dependent variable both before and after the treatment. It is unlike the pretest-posttest design, however, in that it includes multiple pretest and posttest measurements.

Figure 7.3 shows data from a hypothetical interrupted time-series study. The dependent variable is the number of student absences per week in a research methods course. The treatment is that the instructor begins publicly taking attendance each day so that students know that the instructor is aware of who is present and who is absent. The top panel of  Figure 7.3 shows how the data might look if this treatment worked. There is a consistently high number of absences before the treatment, and there is an immediate and sustained drop in absences after the treatment. The bottom panel of  Figure 7.3 shows how the data might look if this treatment did not work. On average, the number of absences after the treatment is about the same as the number before. This figure also illustrates an advantage of the interrupted time-series design over a simpler pretest-posttest design. If there had been only one measurement of absences before the treatment at Week 7 and one afterward at Week 8, then it would have looked as though the treatment were responsible for the reduction. The multiple measurements both before and after the treatment suggest that the reduction between Weeks 7 and 8 is nothing more than normal week-to-week variation.

Combination Designs

A type of quasi-experimental design that is generally better than either the nonequivalent groups design or the pretest-posttest design is one that combines elements of both. There is a treatment group that is given a pretest, receives a treatment, and then is given a posttest. But at the same time there is a control group that is given a pretest, does  not  receive the treatment, and then is given a posttest. The question, then, is not simply whether participants who receive the treatment improve but whether they improve  more  than participants who do not receive the treatment.

Imagine, for example, that students in one school are given a pretest on their attitudes toward drugs, then are exposed to an antidrug program, and finally are given a posttest. Students in a similar school are given the pretest, not exposed to an antidrug program, and finally are given a posttest. Again, if students in the treatment condition become more negative toward drugs, this change in attitude could be an effect of the treatment, but it could also be a matter of history or maturation. If it really is an effect of the treatment, then students in the treatment condition should become more negative than students in the control condition. But if it is a matter of history (e.g., news of a celebrity drug overdose) or maturation (e.g., improved reasoning), then students in the two conditions would be likely to show similar amounts of change. This type of design does not completely eliminate the possibility of confounding variables, however. Something could occur at one of the schools but not the other (e.g., a student drug overdose), so students at the first school would be affected by it while students at the other school would not.

Finally, if participants in this kind of design are randomly assigned to conditions, it becomes a true experiment rather than a quasi experiment. In fact, it is the kind of experiment that Eysenck called for—and that has now been conducted many times—to demonstrate the effectiveness of psychotherapy.

Key Takeaways

• Quasi-experimental research involves the manipulation of an independent variable without the random assignment of participants to conditions or orders of conditions. Among the important types are nonequivalent groups designs, pretest-posttest, and interrupted time-series designs.
• Quasi-experimental research eliminates the directionality problem because it involves the manipulation of the independent variable. It does not eliminate the problem of confounding variables, however, because it does not involve random assignment to conditions. For these reasons, quasi-experimental research is generally higher in internal validity than correlational studies but lower than true experiments.
• Practice: Imagine that two professors decide to test the effect of giving daily quizzes on student performance in a statistics course. They decide that Professor A will give quizzes but Professor B will not. They will then compare the performance of students in their two sections on a common final exam. List five other variables that might differ between the two sections that could affect the results.
• regression to the mean
• spontaneous remission

Image Descriptions

Figure 7.3 image description: Two line graphs charting the number of absences per week over 14 weeks. The first 7 weeks are without treatment and the last 7 weeks are with treatment. In the first line graph, there are between 4 to 8 absences each week. After the treatment, the absences drop to 0 to 3 each week, which suggests the treatment worked. In the second line graph, there is no noticeable change in the number of absences per week after the treatment, which suggests the treatment did not work. [Return to Figure 7.3]

• Cook, T. D., & Campbell, D. T. (1979). Quasi-experimentation: Design & analysis issues in field settings . Boston, MA: Houghton Mifflin. ↵
• Posternak, M. A., & Miller, I. (2001). Untreated short-term course of major depression: A meta-analysis of studies using outcomes from studies using wait-list control groups. Journal of Affective Disorders, 66 , 139–146. ↵
• Eysenck, H. J. (1952). The effects of psychotherapy: An evaluation. Journal of Consulting Psychology, 16 , 319–324. ↵
• Smith, M. L., Glass, G. V., & Miller, T. I. (1980). The benefits of psychotherapy . Baltimore, MD: Johns Hopkins University Press. ↵

A between-subjects design in which participants have not been randomly assigned to conditions.

The dependent variable is measured once before the treatment is implemented and once after it is implemented.

A category of alternative explanations for differences between scores such as events that happened between the pretest and posttest, unrelated to the study.

An alternative explanation that refers to how the participants might have changed between the pretest and posttest in ways that they were going to anyway because they are growing and learning.

The statistical fact that an individual who scores extremely on a variable on one occasion will tend to score less extremely on the next occasion.

The tendency for many medical and psychological problems to improve over time without any form of treatment.

A set of measurements taken at intervals over a period of time that are interrupted by a treatment.

Research Methods in Psychology - 2nd Canadian Edition Copyright © 2015 by Paul C. Price, Rajiv Jhangiani, & I-Chant A. Chiang is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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5 Chapter 5: Experimental and Quasi-Experimental Designs

Case stu dy: the impact of teen court.

Research Study

An Experimental Evaluation of Teen Courts 1

Research Question

Is teen court more effective at reducing recidivism and improving attitudes than traditional juvenile justice processing?

Methodology

Researchers randomly assigned 168 juvenile offenders ages 11 to 17 from four different counties in Maryland to either teen court as experimental group members or to traditional juvenile justice processing as control group members. (Note: Discussion on the technical aspects of experimental designs, including random assignment, is found in detail later in this chapter.) Of the 168 offenders, 83 were assigned to teen court and 85 were assigned to regular juvenile justice processing through random assignment. Of the 83 offenders assigned to the teen court experimental group, only 56 (67%) agreed to participate in the study. Of the 85 youth randomly assigned to normal juvenile justice processing, only 51 (60%) agreed to participate in the study.

Upon assignment to teen court or regular juvenile justice processing, all offenders entered their respective sanction. Approximately four months later, offenders in both the experimental group (teen court) and the control group (regular juvenile justice processing) were asked to complete a post-test survey inquiring about a variety of behaviors (frequency of drug use, delinquent behavior, variety of drug use) and attitudinal measures (social skills, rebelliousness, neighborhood attachment, belief in conventional rules, and positive self-concept). The study researchers also collected official re-arrest data for 18 months starting at the time of offender referral to juvenile justice authorities.

Teen court participants self-reported higher levels of delinquency than those processed through regular juvenile justice processing. According to official re-arrests, teen court youth were re-arrested at a higher rate and incurred a higher average number of total arrests than the control group. Teen court offenders also reported significantly lower scores on survey items designed to measure their �belief in conventional rules� compared to offenders processed through regular juvenile justice avenues. Other attitudinal and opinion measures did not differ significantly between the experimental and control group members based on their post-test responses. In sum, those youth randomly assigned to teen court fared worse than control group members who were not randomly assigned to teen court.

Limitations with the Study Procedure

Limitations are inherent in any research study and those research efforts that utilize experimental designs are no exception. It is important to consider the potential impact that a limitation of the study procedure could have on the results of the study.

In the current study, one potential limitation is that teen courts from four different counties in Maryland were utilized. Because of the diversity in teen court sites, it is possible that there were differences in procedure between the four teen courts and such differences could have impacted the outcomes of this study. For example, perhaps staff members at one teen court were more punishment-oriented than staff members at the other county teen courts. This philosophical difference may have affected treatment delivery and hence experimental group members� belief in conventional attitudes and recidivism. Although the researchers monitored each teen court to help ensure treatment consistency between study sites, it is possible that differences existed in the day-to-day operation of the teen courts that may have affected participant outcomes. This same limitation might also apply to control group members who were sanctioned with regular juvenile justice processing in four different counties.

A researcher must also consider the potential for differences between the experimental and control group members. Although the offenders were randomly assigned to the experimental or control group, and the assumption is that the groups were equivalent to each other prior to program participation, the researchers in this study were only able to compare the experimental and control groups on four variables: age, school grade, gender, and race. It is possible that the experimental and control group members differed by chance on one or more factors not measured or available to the researchers. For example, perhaps a large number of teen court members experienced problems at home that can explain their more dismal post-test results compared to control group members without such problems. A larger sample of juvenile offenders would likely have helped to minimize any differences between the experimental and control group members. The collection of additional information from study participants would have also allowed researchers to be more confident that the experimental and control group members were equivalent on key pieces of information that could have influenced recidivism and participant attitudes.

Finally, while 168 juvenile offenders were randomly assigned to either the experimental or control group, not all offenders agreed to participate in the evaluation. Remember that of the 83 offenders assigned to the teen court experimental group, only 56 (67%) agreed to participate in the study. Of the 85 youth randomly assigned to normal juvenile justice processing, only 51 (60%) agreed to participate in the study. While this limitation is unavoidable, it still could have influenced the study. Perhaps those 27 offenders who declined to participate in the teen court group differed significantly from the 56 who agreed to participate. If so, it is possible that the differences among those two groups could have impacted the results of the study. For example, perhaps the 27 youths who were randomly assigned to teen court but did not agree to be a part of the study were some of the least risky of potential teen court participants�less serious histories, better attitudes to begin with, and so on. In this case, perhaps the most risky teen court participants agreed to be a part of the study, and as a result of being more risky, this led to more dismal delinquency outcomes compared to the control group at the end of each respective program. Because parental consent was required for the study authors to be able to compare those who declined to participate in the study to those who agreed, it is unknown if the participants and nonparticipants differed significantly on any variables among either the experimental or control group. Moreover, of the resulting 107 offenders who took part in the study, only 75 offenders accurately completed the post-test survey measuring offending and attitudinal outcomes.

Again, despite the experimental nature of this study, such limitations could have impacted the study results and must be considered.

Impact on Criminal Justice

Teen courts are generally designed to deal with nonserious first time offenders before they escalate to more serious and chronic delinquency. Innovative programs such as �Scared Straight� and juvenile boot camps have inspired an increase in teen court programs across the country, although there is little evidence regarding their effectiveness compared to traditional sanctions for youthful offenders. This study provides more specific evidence as to the effectiveness of teen courts relative to normal juvenile justice processing. Researchers learned that teen court participants fared worse than those in the control group. The potential labeling effects of teen court, including stigma among peers, especially where the offense may have been very minor, may be more harmful than doing less or nothing. The real impact of this study lies in the recognition that teen courts and similar sanctions for minor offenders may do more harm than good.

One important impact of this study is that it utilized an experimental design to evaluate the effectiveness of a teen court compared to traditional juvenile justice processing. Despite the study�s limitations, by using an experimental design it improved upon previous teen court evaluations by attempting to ensure any results were in fact due to the treatment, not some difference between the experimental and control group. This study also utilized both official and self-report measures of delinquency, in addition to self-report measures on such factors as self-concept and belief in conventional rules, which have been generally absent from teen court evaluations. The study authors also attempted to gauge the comparability of the experimental and control groups on factors such as age, gender, and race to help make sure study outcomes were attributable to the program, not the participants.

In This Chapter You Will Learn

The four components of experimental and quasi-experimental research designs and their function in answering a research question

The differences between experimental and quasi-experimental designs

The importance of randomization in an experimental design

The types of questions that can be answered with an experimental or quasi-experimental research design

About the three factors required for a causal relationship

That a relationship between two or more variables may appear causal, but may in fact be spurious, or explained by another factor

That experimental designs are relatively rare in criminal justice and why

About common threats to internal validity or alternative explanations to what may appear to be a causal relationship between variables

Why experimental designs are superior to quasi-experimental designs for eliminating or reducing the potential of alternative explanations

Introduction

The teen court evaluation that began this chapter is an example of an experimental design. The researchers of the study wanted to determine whether teen court was more effective at reducing recidivism and improving attitudes compared to regular juvenile justice case processing. In short, the researchers were interested in the relationship between variables �the relationship of teen court to future delinquency and other outcomes. When researchers are interested in whether a program, policy, practice, treatment, or other intervention impacts some outcome, they often utilize a specific type of research method/design called experimental design. Although there are many types of experimental designs, the foundation for all of them is the classic experimental design. This research design, and some typical variations of this experimental design, are the focus of this chapter.

Although the classic experiment may be appropriate to answer a particular research question, there are barriers that may prevent researchers from using this or another type of experimental design. In these situations, researchers may turn to quasi-experimental designs. Quasi-experiments include a group of research designs that are missing a key element found in the classic experiment and other experimental designs (hence the term �quasi� experiment). Despite this missing part, quasi-experiments are similar in structure to experimental designs and are used to answer similar types of research questions. This chapter will also focus on quasi-experiments and how they are similar to and different from experimental designs.

Uncovering the relationship between variables, such as the impact of teen court on future delinquency, is important in criminal justice and criminology, just as it is in other scientific disciplines such as education, biology, and medicine. Indeed, whereas criminal justice researchers may be interested in whether a teen court reduces recidivism or improves attitudes, medical field researchers may be concerned with whether a new drug reduces cholesterol, or an education researcher may be focused on whether a new teaching style leads to greater academic gains. Across these disciplines and topics of interest, the experimental design is appropriate. In fact, experimental designs are used in all scientific disciplines; the only thing that changes is the topic. Specific to criminal justice, below is a brief sampling of the types of questions that can be addressed using an experimental design:

Does participation in a correctional boot camp reduce recidivism?

What is the impact of an in-cell integration policy on inmate-on-inmate assaults in prisons?

Does police officer presence in schools reduce bullying?

Do inmates who participate in faith-based programming while in prison have a lower recidivism rate upon their release from prison?

Do police sobriety checkpoints reduce drunken driving fatalities?

What is the impact of a no-smoking policy in prisons on inmate-on-inmate assaults?

Does participation in a domestic violence intervention program reduce repeat domestic violence arrests?

A focus on the classic experimental design will demonstrate the usefulness of this research design for addressing criminal justice questions interested in cause and effect relationships. Particular attention is paid to the classic experimental design because it serves as the foundation for all other experimental and quasi-experimental designs, some of which are covered in this chapter. As a result, a clear understanding of the components, organization, and logic of the classic experimental design will facilitate an understanding of other experimental and quasi-experimental designs examined in this chapter. It will also allow the reader to better understand the results produced from those various designs, and importantly, what those results mean. It is a truism that the results of a research study are only as �good� as the design or method used to produce them. Therefore, understanding the various experimental and quasi-experimental designs is the key to becoming an informed consumer of research.

The Challenge of Establishing Cause and Effect

Researchers interested in explaining the relationship between variables, such as whether a treatment program impacts recidivism, are interested in causation or causal relationships. In a simple example, a causal relationship exists when X (independent variable) causes Y (dependent variable), and there are no other factors (Z) that can explain that relationship. For example, offenders who participated in a domestic violence intervention program (X�domestic violence intervention program) experienced fewer re-arrests (Y�re-arrests) than those who did not participate in the domestic violence program, and no other factor other than participation in the domestic violence program can explain these results. The classic experimental design is superior to other research designs in uncovering a causal relationship, if one exists. Before a causal relationship can be established, however, there are three conditions that must be met (see Figure 5.1). 2

FIGURE 5.1 | The Cause and Effect Relationship

Timing The first condition for a causal relationship is timing. For a causal relationship to exist, it must be shown that the independent variable or cause (X) preceded the dependent variable or outcome (Y) in time. A decrease in domestic violence re-arrests (Y) cannot occur before participation in a domestic violence reduction program (X ), if the domestic violence program is proposed to be the cause of fewer re-arrests. Ensuring that cause comes before effect is not sufficient to establish that a causal relationship exists, but it is one requirement that must be met for a causal relationship.

Association In addition to timing, there must also be an observable association between X and Y, the second necessary condition for a causal relationship. Association is also commonly referred to as covariance or correlation. When an association or correlation exits, this means there is some pattern of relationship between X and Y �as X changes by increasing or decreasing, Y also changes by increasing or decreasing. Here, the notion of X and Y increasing or decreasing can mean an actual increase/decrease in the quantity of some factor, such as an increase/decrease in the number of prison terms or days in a program or re-arrests. It can also refer to an increase/decrease in a particular category, for example, from nonparticipation in a program to participation in a program. For instance, subjects who participated in a domestic violence reduction program (X) incurred fewer domestic violence re-arrests (Y) than those who did not participate in the program. In this example, X and Y are associated�as X change s or increases from nonparticipation to participation in the domestic violence program, Y or the number of re-arrests for domestic violence decreases.

Associations between X and Y can occur in two different directions: positive or negative. A positive association means that as X increases, Y increases, or, as X decreases, Y decreases. A negative association means that as X increases, Y decreases, or, as X decreases, Y increases. In the example above, the association is negative�participation in the domestic violence program was associated with a reduction in re-arrests. This is also sometimes called an inverse relationship.

Elimination of Alternative Explanations Although participation in a domestic violence program may be associated with a reduction in re-arrests, this does not mean for certain that participation in the program was the cause of reduced re-arrests. Just as timing by itself does not imply a causal relationship, association by itself does not imply a causal relationship. For example, instead of the program being the cause of a reduction in re-arrests, perhaps several of the program participants died shortly after completion of the domestic violence program and thus were not able to engage in domestic violence (and their deaths were unknown to the researcher tracking re-arrests). Perhaps a number of the program participants moved out of state and domestic violence re-arrests occurred but were not able to be uncovered by the researcher. Perhaps those in the domestic violence program experienced some other event, such as the trauma of a natural disaster, and that experience led to a reduction in domestic violence, an event not connected to the domestic violence program. If any of these situations occurred, it might appear that the domestic violence program led to fewer re-arrests. However, the observed reduction in re-arrests can actually be attributed to a factor unrelated to the domestic violence program.

The previous discussion leads to the third and final necessary consideration in determining a causal relationship� elimination of alternative explanations. This means that the researcher must rule out any other potential explanation of the results, except for the experimental condition such as a program, policy, or practice. Accounting for or ruling out alternative explanations is much more difficult than ensuring timing and association. Ruling out all alternative explanations is difficult because there are so many potential other explanations that can wholly or partly explain the findings of a research study. This is especially true in the social sciences, where researchers are often interested in relationships explaining human behavior. Because of this difficulty, associations by themselves are sometimes mistaken as causal relationships when in fact they are spurious. A spurious relationship is one where it appears that X and Y are causally related, but the relationship is actually explained by something other than the independent variable, or X.

One only needs to go so far as the daily newspaper to find headlines and stories of mere associations being mistaken, assumed, or represented as causal relationships. For example, a newspaper headline recently proclaimed �Churchgoers live longer.� 3 An uninformed consumer may interpret this headline as evidence of a causal relationship�that going to church by itself will lead to a longer life�but the astute consumer would note possible alternative explanations. For example, people who go to church may live longer because they tend to live healthier lifestyles and tend to avoid risky situations. These are two probable alternative explanations to the relationship independent of simply going to church. In another example, researchers David Kalist and Daniel Yee explored the relationship between first names and delinquent behavior in their manuscript titled �First Names and Crime: Does Unpopularity Spell Trouble?� 4 Kalist and Lee (2009) found that unpopular names are associated with juvenile delinquency. In other words, those individuals with the most unpopular names were more likely to be delinquent than those with more popular names. According to the authors, is it not necessarily someone�s name that leads to delinquent behavior, but rather, the most unpopular names also tend to be correlated with individuals who come from disadvantaged home environments and experience a low socio-economic status of living. Rightly noted by the authors, these alternative explanations help to explain the link between someone�s name and delinquent behavior�a link that is not causal.

A frequently cited example provides more insight to the claim that an association by itself is not sufficient to prove causality. In certain cities in the United States, for example, as ice cream sales increase on a particular day or in a particular month so does the incidence of certain forms of crime. If this association were represented as a causal statement, it would be that ice cream or ice cream sales causes crime. There is an association, no doubt, and let us assume that ice cream sales rose before the increase in crime (timing). Surely, however, this relationship between ice cream sales and crime is spurious. The alternative explanation is that ice cream sales and crime are associated in certain parts of the country because of the weather. Ice cream sales tend to increase in warmer temperatures, and it just so happens that certain forms of crime tend to increase in warmer temperatures as well. This coincidence or association does not mean a causal relationship exists. Additionally, this does not mean that warm temperatures cause crime either. There are plenty of other alternative explanations for the increase in certain forms of crime and warmer temperatures. 6 For another example of a study subject to alternative explanations, read the June 2011 news article titled �Less Crime in U.S. Thanks to Videogames.� 7 Based on your reading, what are some other potential explanations for the crime drop other than videogames?

The preceding examples demonstrate how timing and association can be present, but the final needed condition for a causal relationship is that all alternative explanations are ruled out. While this task is difficult, the classic experimental design helps to ensure these additional explanatory factors are minimized. When other designs are used, such as quasi-experimental designs, the chance that alternative explanations emerge is greater. This potential should become clearer as we explore the organization and logic of the classic experimental design.

CLASSICS IN CJ RESEARCH

Minneapolis Domestic Violence Experiment

The Minneapolis Domestic Violence Experiment (MDVE) 5

Which police action (arrest, separation, or mediation) is most effective at deterring future misdemeanor domestic violence?

The experiment began on March 17, 1981, and continued until August 1, 1982. The experiment was conducted in two of Minneapolis�s four police precincts�the two with the highest number of domestic violence reports and arrests. A total of 314 reports of misdemeanor domestic violence were handled by the police during this time frame.

This study utilized an experimental design with the random assignment of police actions. Each police officer involved in the study was given a pad of report forms. Upon a misdemeanor domestic violence call, the officer�s action (arrest, separation, or mediation) was predetermined by the order and color of report forms in the officer�s notebook. Colored report forms were randomly ordered in the officer�s notebook and the color on the form determined the officer response once at the scene. For example, after receiving a call for domestic violence, an officer would turn to his or her report pad to determine the action. If the top form was pink, the action was arrest. If on the next call the top form was a different color, an action other than arrest would occur. All colored report forms were randomly ordered through a lottery assignment method. The result is that all police officer actions to misdemeanor domestic violence calls were randomly assigned. To ensure the lottery procedure was properly carried out, research staff participated in ride-alongs with officers to ensure that officers did not skip the order of randomly ordered forms. Research staff also made sure the reports were received in the order they were randomly assigned in the pad of report forms.

To examine the relationship of different officer responses to future domestic violence, the researchers examined official arrests of the suspects in a 6-month follow-up period. For example, the researchers examined those initially arrested for misdemeanor domestic violence and how many were subsequently arrested for domestic violence within a 6-month time frame. They did the same procedure for the police actions of separation and mediation. The researchers also interviewed the victim(s) of each incident and asked if a repeat domestic violence incident occurred with the same suspect in the 6-month follow-up period. This allowed researchers to examine domestic violence offenses that may have occurred but did not come to the official attention of police. The researchers then compared official arrests for domestic violence to self-reported domestic violence after the experiment.

Suspects arrested for misdemeanor domestic violence, as opposed to situations where separation or mediation was used, were significantly less likely to engage in repeat domestic violence as measured by official arrest records and victim interviews during the 6-month follow-up period. According to official police records, 10% of those initially arrested engaged in repeat domestic violence in the followup period, 19% of those who initially received mediation engaged in repeat domestic violence, and 24% of those who randomly received separation engaged in repeat domestic violence. According to victim interviews, 19% of those initially arrested engaged in repeat domestic violence, compared to 37% for separation and 33% for mediation. The general conclusion of the experiment was that arrest was preferable to separation or mediation in deterring repeat domestic violence across both official police records and victim interviews.

A few issues that affected the random assignment procedure occurred throughout the study. First, some officers did not follow the randomly assigned action (arrest, separation, or mediation) as a result of other circumstances that occurred at the scene. For example, if the randomly assigned action was separation, but the suspect assaulted the police officer during the call, the officer might arrest the suspect. Second, some officers simply ignored the assigned action if they felt a particular call for domestic violence required another action. For example, if the action was mediation as indicated by the randomly assigned report form, but the officer felt the suspect should be arrested, he or she may have simply ignored the randomly assigned response and substituted his or her own. Third, some officers forgot their report pads and did not know the randomly assigned course of action to take upon a call of domestic violence. Fourth and finally, the police chief also allowed officers to deviate from the randomly assigned action in certain circumstances. In all of these situations, the random assignment procedures broke down.

The results of the MDVE had a rapid and widespread impact on law enforcement practice throughout the United States. Just two years after the release of the study, a 1986 telephone survey of 176 urban police departments serving cities with populations of 100,000 or more found that 46 percent of the departments preferred to make arrests in cases of minor domestic violence, largely due to the effectiveness of this practice in the Minneapolis Domestic Violence Experiment. 8

In an attempt to replicate the findings of the Minneapolis Domestic Violence Experiment, the National Institute of Justice sponsored the Spouse Assault Replication Program. Replication studies were conducted in Omaha, Charlotte, Milwaukee, Miami, and Colorado Springs from 1986�1991. In three of the five replications, offenders randomly assigned to the arrest group had higher levels of continued domestic violence in comparison to other police actions during domestic violence situations. 9 Therefore, rather than providing results that were consistent with the Minneapolis Domestic Violence Experiment, the results from the five replication experiments produced inconsistent findings about whether arrest deters domestic violence. 10

Despite the findings of the replications, the push to arrest domestic violence offenders has continued in law enforcement. Today many police departments require officers to make arrests in domestic violence situations. In agencies that do not mandate arrest, department policy typically states a strong preference toward arrest. State legislatures have also enacted laws impacting police actions regarding domestic violence. Twenty-one states have mandatory arrest laws while eight have pro-arrest statutes for domestic violence. 11

The Classic Experimental Design

Table 5.1 provides an illustration of the classic experimental design. 12 It is important to become familiar with the specific notation and organization of the classic experiment before a full discussion of its components and their purpose.

Major Components of the Classic Experimental Design

The classic experimental design has four major components:

1. Treatment

2. Experimental Group and Control Group

3. Pre-Test and Post-Test

4. Random Assignment

Treatment The first component of the classic experimental design is the treatment, and it is denoted by X in the classic experimental design. The treatment can be a number of things�a program, a new drug, or the implementation of a new policy. In a classic experimental design, the primary goal is to determine what effect, if any, a particular treatment had on some outcome. In this way, the treatment can also be considered the independent variable.

TABLE 5.1 | The Classic Experimental Design

Experimental Group = Group that receives the treatment

Control Group = Group that does not receive the treatment

R = Random assignment

O 1 = Observation before the treatment, or the pre-test

X = Treatment or the independent variable

O 2 = Observation after the treatment, or the post-test

Experimental and Control Groups The second component of the classic experiment is an experimental group and a control group. The experimental group receives the treatment, and the control group does not receive the treatment. There will always be at least one group that receives the treatment in experimental and quasi-experimental designs. In some cases, experiments may have multiple experimental groups receiving multiple treatments.

Pre-Test and Post-Test The third component of the classic experiment is a pre-test and a post-test. A pretest is a measure of the dependent variable or outcome before the treatment. The post-test is a measure of the dependent variable after the treatment is administered. It is important to note that the post-test is defined based on the stated goals of the program. For example, if the stated goal of a particular program is to reduce re-arrests, the post-test will be a measure of re-arrests after the program. The dependent variable also defines the pre-test. For example, if a researcher wanted to examine the impact of a domestic violence reduction program (treatment or X) on the goal of reducing re-arrests (dependent variable or Y), the pre-test would be the number of domestic violence arrests incurred before the program. Program goals may be numerous and all can constitute a post-test, and hence, the pre-test. For example, perhaps the goal of the domestic violence program is also that participants learn of different pro-social ways to handle domestic conflicts other than resorting to violence. If researchers wanted to examine this goal, the post-test might be subjects� level of knowledge about pro-social ways to handle domestic conflicts other than violence. The pre-test would then be subjects� level of knowledge about these pro-social alternatives to violence before they received the treatment program.

Although all designs have a post-test, it is not always the case that designs have a pre-test. This is because researchers may not have access or be able to collect information constituting the pre-test. For example, researchers may not be able to determine subjects� level of knowledge about alternatives to domestic violence before the intervention program if the subjects are already enrolled in the domestic violence intervention program. In other cases, there may be financial barriers to collecting pre-test information. In the teen court evaluation that started this chapter, for example, researchers were not able to collect pre-test information on study participants due to the financial strain it would have placed on the agencies involved in the study. 13 There are a number of potential reasons why a pre-test might not be available in a research study. The defining feature, however, is that the pre-test is determined by the post-test.

Random Assignment The fourth component of the classic experiment is random assignment. Random assignment refers to a process whereby members of the experimental group and control group are assigned to the two groups through a random and unbiased process. Random assignment should not be mistaken for random selection as discussed in Chapter 3. Random selection refers to selecting a smaller but representative sample from a larger population. For example, a researcher may randomly select a sample from a larger city population for the purposes of sending sample members a mail survey to determine their attitudes on crime. The goal of random selection in this example is to make sure the sample, although smaller in size than the population, accurately represents the larger population.

Random assignment, on the other hand, refers to the process of assigning subjects to either the experimental or control group with the goal that the groups are similar or equivalent to each other in every way (see Figure 5.2). The exception to this rule is that one group gets the treatment and the other does not (see discussion below on why equivalence is so important). Although the concept of random is similar in each, the goals are different between random selection and random assignment. 14 Experimental designs all feature random assignment, but this is not true of other research designs, in particular quasi-experimental designs.

FIGURE 5.2 | Random Assignment

The classic experimental design is the foundation for all other experimental and quasi-experimental designs because it retains all of the major components discussed above. As mentioned, sometimes designs do not have a pre-test, a control group, or random assignment. Because the pre-test, control group, and random assignment are so critical to the goal of uncovering a causal relationship, if one exists, we explore them further below.

The Logic of the Classic Experimental Design

Consider a research study using the classic experimental design where the goal is to determine if a domestic violence treatment program has any effect on re-arrests for domestic violence. The randomly assigned experimental and control groups are comprised of persons who had previously been arrested for domestic violence. The pretest is a measure of the number of domestic violence arrests before the program. This is because the goal of the program is to determine whether re-arrests are impacted after the treatment. The post-test is the number of re-arrests following the treatment program.

Once randomly assigned, the experimental group members receive the domestic violence program, and the control group members do not. After the program, the researcher will compare the pre-test arrests for domestic violence of the experimental group to post-test arrests for domestic violence to determine if arrests increased, decreased, or remained constant since the start of the program. The researcher will also compare the post-test re-arrests for domestic violence between the experimental and control groups. With this example, we explore the usefulness of the classic experimental design, and the contribution of the pre-test, random assignment, and the control group to the goal of determining whether a domestic violence program reduces re-arrests.

The Pre-Test As a component of the classic experiment, the pre-test allows an examination of change in the dependent variable from before the domestic violence program to after the domestic violence program. In short, a pre-test allows the researcher to determine if re-arrests increased, decreased, or remained the same following the domestic violence program. Without a pre-test, researchers would not be able to determine the extent of change, if any, from before to after the program for either the experimental or control group.

Although the pre-test is a measure of the dependent variable before the treatment, it can also be thought of as a measure whereby the researcher can compare the experimental group to the control group before the treatment is administered. For example, the pre-test helps researchers to make sure both groups are similar or equivalent on previous arrests for domestic violence. The importance of equivalence between the experimental and control groups on previous arrests is discussed below with random assignment.

Random Assignment Random assignment helps to ensure that the experimental and control groups are equivalent before the introduction of the treatment. This is perhaps one of the most critical aspects of the classic experiment and all experimental designs. Although the experimental and control groups will be made up of different people with different characteristics, assigning them to groups via a random assignment process helps to ensure that any differences or bias between the groups is eliminated or minimized. By minimizing bias, we mean that the groups will balance each other out on all factors except the treatment. If they are balanced out on all factors prior to the administration of the treatment, any differences between the groups at the post-test must be due to the treatment�the only factor that differs between the experimental group and the control group. According to Shadish, Cook, and Campbell: �If implemented correctly, random assignment creates two or more groups of units that are probabilistically similar to each other on the average. Hence, any outcome differences that are observed between those groups at the end of a study are likely to be due to treatment, not to differences between the groups that already existed at the start of the study.� 15 Considered in another way, if the experimental and control group differed significantly on any relevant factor other than the treatment, the researcher would not know if the results observed at the post-test are attributable to the treatment or to the differences between the groups.

Consider an example where 500 domestic abusers were randomly assigned to the experimental group and 500 were randomly assigned to the control group. Because they were randomly assigned, we would likely find more frequent domestic violence arrestees in both groups, older and younger arrestees in both groups, and so on. If random assignment was implemented correctly, it would be highly unlikely that all of the experimental group members were the most serious or frequent arrestees and all of the control group members were less serious and/or less frequent arrestees. While there are no guarantees, we know the chance of this happening is extremely small with random assignment because it is based on known probability theory. Thus, except for a chance occurrence, random assignment will result in equivalence between the experimental and control group in much the same way that flipping a coin multiple times will result in heads approximately 50% of the time and tails approximately 50% of the time. Over 1,000 tosses of a coin, for example, should result in roughly 500 heads and 500 tails. While there is a chance that flipping a coin 1,000 times will result in heads 1,000 times, or some other major imbalance between heads and tails, this potential is small and would only occur by chance.

The same logic from above also applies with randomly assigning people to groups, and this can even be done by flipping a coin. By assigning people to groups through a random and unbiased process, like flipping a coin, only by chance (or researcher error) will one group have more of one characteristic than another, on average. If there are no major (also called statistically significant) differences between the experimental and control group before the treatment, the most plausible explanation for the results at the post-test is the treatment.

As mentioned, it is possible by some chance occurrence that the experimental and control group members are significantly different on some characteristic prior to administration of the treatment. To confirm that the groups are in fact similar after they have been randomly assigned, the researcher can examine the pre-test if one is present. If the researcher has additional information on subjects before the treatment is administered, such as age, or any other factor that might influence post-test results at the end of the study, he or she can also compare the experimental and control group on those measures to confirm that the groups are equivalent. Thus, a researcher can confirm that the experimental and control groups are equivalent on information known to the researcher.

Being able to compare the groups on known measures is an important way to ensure the random assignment process �worked.� However, perhaps most important is that randomization also helps to ensure similarity across unknown variables between the experimental and control group. Because random assignment is based on known probability theory, there is a much higher probability that all potential differences between the groups that could impact the post-test should balance out with random assignment�known or unknown. Without random assignment, it is likely that the experimental and control group would differ on important but unknown factors and such differences could emerge as alternative explanations for the results. For example, if a researcher did not utilize random assignment and instead took the first 500 domestic abusers from an ordered list and assigned them to the experimental group and the last 500 domestic abusers and assigned them to the control group, one of the groups could be �lopsided� or imbalanced on some important characteristic that could impact the outcome of the study. With random assignment, there is a much higher likelihood that these important characteristics among the experimental and control groups will balance out because no individual has a different chance of being placed into one group versus the other. The probability of one or more characteristics being concentrated into one group and not the other is extremely small with random assignment.

To further illustrate the importance of random assignment to group equivalence, suppose the first 500 domestic violence abusers who were assigned to the experimental group from the ordered list had significantly fewer domestic violence arrests before the program than the last 500 domestic violence abusers on the list. Perhaps this is because the ordered list was organized from least to most chronic domestic abusers. In this instance, the control group would be lopsided concerning number of pre-program domestic violence arrests�they would be more chronic than the experimental group. The arrest imbalance then could potentially explain the post-test results following the domestic violence program. For example, the �less risky� offenders in the experimental group might be less likely to be re-arrested regardless of their participation in the domestic violence program, especially compared to the more chronic domestic abusers in the control group. Because of imbalances between the experimental and control group on arrests before the program was implemented, it would not be known for certain whether an observed reduction in re-arrests after the program for the experimental group was due to the program or the natural result of having less risky offenders in the experimental group. In this instance, the results might be taken to suggest that the program significantly reduces re-arrests. This conclusion might be spurious, however, for the association may simply be due to the fact that the offenders in the experimental group were much different (less frequent offenders) than the control group. Here, the program may have had no effect�the experimental group members may have performed the same regardless of the treatment because they were low-level offenders.

The example above suggests that differences between the experimental and control groups based on previous arrest records could have a major impact on the results of a study. Such differences can arise with the lack of random assignment. If subjects were randomly assigned to the experimental and control group, however, there would be a much higher probability that less frequent and more frequent domestic violence arrestees would have been found in both the experimental and control groups and the differences would have balanced out between the groups�leaving any differences between the groups at the post-test attributable to the treatment only.

In summary, random assignment helps to ensure that the experimental and control group members are balanced or equivalent on all factors that could impact the dependent variable or post-test�known or unknown. The only factor they are not balanced or equal on is the treatment. As such, random assignment helps to isolate the impact of the treatment, if any, on the post-test because it increases confidence that the only difference between the groups should be that one group gets the treatment and the other does not. If that is the only difference between the groups, any change in the dependent variable between the experimental and control group must be attributed to the treatment and not an alternative explanation, such as significant arrest history imbalance between the groups (refer to Figure 5.2). This logic also suggests that if the experimental group and control group are imbalanced on any factor that may be relevant to the outcome, that factor then becomes a potential alternative explanation for the results�an explanation that reduces the researcher�s ability to isolate the real impact of the treatment.

WHAT RESEARCH SHOWS: IMPACTING CRIMINAL JUSTICE OPERATIONS

Scared Straight

The 1978 documentary Scared Straight introduced to the public the �Lifer�s Program� at Rahway State Prison in New Jersey. This program sought to decrease juvenile delinquency by bringing at-risk and delinquent juveniles into the prison where they would be �scared straight� by inmates serving life sentences. Participants in the program were talked to and yelled at by the inmates in an effort to scare them. It was believed that the fear felt by the participants would lead to a discontinuation of their problematic behavior so that they would not end up in prison themselves. Although originally touted as a success based on anecdotal evidence, subsequent evaluations of the program and others like it proved otherwise.

Using a classic experimental design, Finckenauer evaluated the original �Lifer�s Program� at Rahway State Prison. 16 Participating juveniles were randomly assigned to the experimental group or the control group. Results of the evaluation were not positive. Post-test measures revealed that juveniles who were assigned to the experimental group and participated in the program were actually more seriously delinquent afterwards than those who did not participate in the program. Also using an experimental design with random assignment, Yarborough evaluated the �Juvenile Offenders Learn Truth� (JOLT) program at the State Prison of Southern Michigan at Jackson. 17 This program was similar to that of the �Lifer�s Program� only with fewer obscenities used by the inmates. Post-test measurements were taken at two intervals, 3 and 6 months after program completion. Again, results were not positive. Findings revealed no significant differences between those juveniles who attended the program and those who did not.

Other experiments conducted on Scared Straight -like programs further revealed their inability to deter juveniles from future criminality. 18 Despite the intuitive popularity of these programs, these evaluations proved that such programs were not successful. In fact, it is postulated that these programs may have actually done more harm than good.

The Control Group The presence of an equivalent control group (created through random assignment) also gives the researcher more confidence that the findings at the post-test are due to the treatment and not some other alternative explanation. This logic is perhaps best demonstrated by considering how interpretation of results is affected without a control group. Absent an equivalent control group, it cannot be known whether the results of the study are due to the program or some other factor. This is because the control group provides a baseline of comparison or a �control.� For example, without a control group, the researcher may find that domestic violence arrests declined from pre-test to post-test. But the researcher would not be able to definitely attribute that finding to the program without a control group. Perhaps the single experimental group incurred fewer arrests because they matured over their time in the program, regardless of participation in the domestic violence program. Having a randomly assigned control group would allow this consideration to be eliminated, because the equivalent control group would also have naturally matured if that was the case.

Because the control group is meant to be similar to the experimental group on all factors with the exception that the experimental group receives the treatment, the logic is that any differences between the experimental and control group after the treatment must then be attributable only to the treatment itself�everything else occurs equally in both the experimental and control groups and thus cannot be the cause of results. The bottom line is that a control group allows the researcher more confidence to attribute any change in the dependent variable from pre- to post-test and between the experimental and control groups to the treatment�and not another alternative explanation. Absent a control group, the researcher would have much less confidence in the results.

Knowledge about the major components of the classic experimental design and how they contribute to an understanding of cause and effect serves as an important foundation for studying different types of experimental and quasi-experimental designs and their organization. A useful way to become familiar with the components of the experimental design and their important role is to consider the impact on the interpretation of results when one or more components are lacking. For example, what if a design lacked a pre-test? How could this impact the interpretation of post-test results and knowledge about the comparability of the experimental and control group? What if a design lacked random assignment? What are some potential problems that could occur and how could those potential problems impact interpretation of results? What if a design lacked a control group? How does the absence of an equivalent control group affect a researcher�s ability to determine the unique effects of the treatment on the outcomes being measured? The ability to discuss the contribution of a pre-test, random assignment, and a control group�and what is the impact when one or more of those components is absent from a research design�is the key to understanding both experimental and quasi-experimental designs that will be discussed in the remainder of this chapter. As designs lose these important parts and transform from a classic experiment to another experimental design or to a quasi-experiment, they become less useful in isolating the impact that a treatment has on the dependent variable and allow more room for alternative explanations of the results.

One more important point must be made before further delving into experimental and quasi-experimental designs. This point is that rarely, if ever, will the average consumer of research be exposed to the symbols or specific language of the classic experiment, or other experimental and quasi-experimental designs examined in this chapter. In fact, it is unlikely that the average consumer will ever be exposed to the terms pre-test, post-test, experimental group, or random assignment in the popular media, among other terms related to experimental and quasi-experimental designs. Yet, consumers are exposed to research results produced from these and other research designs every day. For example, if a national news organization or your regional newspaper reported a story about the effectiveness of a new drug to reduce cholesterol or the effects of different diets on weight loss, it is doubtful that the results would be reported as produced through a classic experimental design that used a control group and random assignment. Rather, these media outlets would use generally nonscientific terminology such as �results of an experiment showed� or �results of a scientific experiment indicated� or �results showed that subjects who received the new drug had greater cholesterol reductions than those who did not receive the new drug.� Even students who regularly search and read academic articles for use in course papers and other projects will rarely come across such design notation in the research studies they utilize. Depiction of the classic experimental design, including a discussion of its components and their function, simply illustrates the organization and notation of the classic experimental design. Unfortunately, the average consumer has to read between the lines to determine what type of design was used to produce the reported results. Understanding the key components of the classic experimental design allows educated consumers of research to read between those lines.

RESEARCH IN THE NEWS

�Swearing Makes Pain More Tolerable� 19

In 2009, Richard Stephens, John Atkins, and Andrew Kingston of the School of Psychology at Keele University conducted a study with 67 undergraduate students to determine if swearing affects an individual�s response to pain. Researchers asked participants to immerse their hand in a container filled with ice-cold water and repeat a preferred swear word. The researchers then asked the same participants to immerse their hand in ice-cold water while repeating a word used to describe a table (a non-swear word). The results showed that swearing increased pain tolerance compared to the non-swearing condition. Participants who used a swear word were able to hold their hand in ice-cold water longer than when they did not swear. Swearing also decreased participants� perception of pain.

1. This study is an example of a repeated measures design. In this form of experimental design, study participants are exposed to an experimental condition (swearing with hand in ice-cold water) and a control condition (non-swearing with hand in ice-cold water) while repeated outcome measures are taken with each condition, for example, the length of time a participant was able to keep his or her hand submerged in ice-cold water. Conduct an Internet search for �repeated measures design� and explore the various ways such a study could be conducted, including the potential benefits and drawbacks to this design.

2. After researching repeated measures designs, devise a hypothetical repeated measures study of your own.

3. Retrieve and read the full research study �Swearing as a Response to Pain� by Stephens, Atkins, and Kingston while paying attention to the design and methods (full citation information for this study is listed below). Has your opinion of the study results changed after reading the full study? Why or why not?

Full Study Source: Stephens, R., Atkins, J., and Kingston, A. (2009). �Swearing as a response to pain.� NeuroReport 20, 1056�1060.

Variations on the Experimental Design

The classic experimental design is the foundation upon which all experimental and quasi-experimental designs are based. As such, it can be modified in numerous ways to fit the goals (or constraints) of a particular research study. Below are two variations of the experimental design. Again, knowledge about the major components of the classic experiment, how they contribute to an explanation of results, and what the impact is when one or more components are missing provides an understanding of all other experimental designs.

Post-Test Only Experimental Design

The post-test only experimental design could be used to examine the impact of a treatment program on school disciplinary infractions as measured or operationalized by referrals to the principal�s office (see Table 5.2). In this design, the researcher randomly assigns a group of discipline problem students to the experimental group and control group by flipping a coin�heads to the experimental group and tails to the control group. The experimental group then enters the 3-month treatment program. After the program, the researcher compares the number of referrals to the principal�s office between the experimental and control groups over some period of time, for example, discipline referrals at 6 months after the program. The researcher finds that the experimental group has a much lower number of referrals to the principal�s office in the 6 month follow-up period than the control group.

TABLE 5.2 | Post-Test Only Experimental Design

Several issues arise in this example study. The researcher would not know if discipline problems decreased, increased, or stayed the same from before to after the treatment program because the researcher did not have a count of disciplinary referrals prior to the treatment program (e.g., a pre-test). Although the groups were randomly assigned and are presumed equivalent, the absence of a pre-test means the researcher cannot confirm that the experimental and control groups were equivalent before the treatment was administered, particularly on the number of referrals to the principal�s office. The groups could have differed by a chance occurrence even with random assignment, and any such differences between the groups could potentially explain the post-test difference in the number of referrals to the principal�s office. For example, if the control group included much more serious or frequent discipline problem students than the experimental group by chance, this difference might explain the lower number of referrals for the experimental group, not that the treatment produced this result.

Experimental Design with Two Treatments and a Control Group

This design could be used to determine the impact of boot camp versus juvenile detention on post-release recidivism (see Table 5.3). Recidivism in this study is operationalized as re-arrest for delinquent behavior. First, a population of known juvenile delinquents is randomly assigned to either boot camp, juvenile detention, or a control condition where they receive no sanction. To accomplish random assignment to groups, the researcher places the names of all youth into a hat and assigns the groups in order. For example, the first name pulled goes into experimental group 1, the next into experimental group 2, and the next into the control group, and so on. Once randomly assigned, the experimental group youth receive either boot camp or juvenile detention for a period of 3 months, whereas members of the control group are released on their own recognizance to their parents. At the end of the experiment, the researcher compares the re-arrest activity of boot camp participants to detention delinquents to control group members during a 6-month follow-up period.

TABLE 5.3 | Experimental Design with Two Treatments and a Control Group

This design has several advantages. First, it includes all major components of the classic experimental design, and simply adds an additional treatment for comparison purposes. Random assignment was utilized and this means that the groups have a higher probability of being equivalent on all factors that could impact the post-test. Thus, random assignment in this example helps to ensure the only differences between the groups are the treatment conditions. Without random assignment, there is a greater chance that one group of youth was somehow different, and this difference could impact the post-test. For example, if the boot camp youth were much less serious and frequent delinquents than the juvenile detention youth or control group youth, the results might erroneously show that the boot camp reduced recidivism when in fact the youth in boot camp may have been the �best risks��unlikely to get re-arrested with or without boot camp. The pre-test in the example above allows the researcher to determine change in re-arrests from pretest to post-test. Thus, the researcher can determine if delinquent behavior, as measured by re-arrest, increased, decreased, or remained constant from pre- to post-test. The pre-test also allows the researcher to confirm that the random assignment process resulted in equivalent groups based on the pre-test. Finally, the presence of a control group allows the researcher to have more confidence that any differences in the post-test are due to the treatment. For example, if the control group had more re-arrests than the boot camp or juvenile detention experimental groups 6 months after their release from those programs, the researcher would have more confidence that the programs produced fewer re-arrests because the control group members were the same as the experimental groups; the only difference was that they did not receive a treatment.

The one key feature of experimental designs is that they all retain random assignment. This is why they are considered �experimental� designs. Sometimes, however, experimental designs lack a pre-test. Knowledge of the usefulness of a pre-test demonstrates the potential problems with those designs where it is missing. For example, in the post-test only experimental design, a researcher would not be able to make a determination of change in the dependent variable from pre- to post-test. Perhaps most importantly, the researcher would not be able to confirm that the experimental and control groups were in fact equivalent on a pre-test measure before the introduction of the treatment. Even though both groups were randomly assigned, and probability theory suggests they should be equivalent, without a pre-test measure the researcher could not confirm similarity because differences could occur by chance even with random assignment. If there were any differences at the post-test between the experimental group and control group, the results might be due to some explanation other than the treatment, namely that the groups differed prior to the administration of the treatment. The same limitation could apply in any form of experimental design that does not utilize a pre-test for conformational purposes.

Understanding the contribution of a pre-test to an experimental design shows that it is a critical component. It provides a measure of change and also gives the researcher more confidence that the observed results are due to the treatment, and not some difference between the experimental and control groups. Despite the usefulness of a pre-test, however, perhaps the most critical ingredient of any experimental design is random assignment. It is important to note that all experimental designs retain random assignment.

Experimental Designs Are Rare in Criminal Justice and Criminology

The classic experiment is the foundation for other types of experimental and quasi-experimental designs. The unfortunate reality, however, is that the classic experiment, or other experimental designs, are few and far between in criminal justice. 20 Recall that one of the major components of an experimental design is random assignment. Achieving random assignment is often a barrier to experimental research in criminal justice. Achieving random assignment might, for example, require the approval of the chief (or city council or both) of a major metropolitan police agency to allow researchers to randomly assign patrol officers to certain areas of a city and/or randomly assign police officer actions. Recall the MDVE. This experiment required the full cooperation of the chief of police and other decision-makers to allow researchers to randomly assign police actions. In another example, achieving random assignment might require a judge to randomly assign a group of youthful offenders to a certain juvenile court sanction (experimental group), and another group of similar youthful offenders to no sanction or an alternative sanction as a control group. 21 In sum, random assignment typically requires the cooperation of a number of individuals and sometimes that cooperation is difficult to obtain.

Even when random assignment can be accomplished, sometimes it is not implemented correctly and the random assignment procedure breaks down. This is another barrier to conducting experimental research. For example, in the MDVE, researchers randomly assigned officer responses, but the officers did not always follow the assigned course of action. Moreover, some believe that the random assignment of criminal justice programs, sentences, or randomly assigning officer responses may be unethical in certain circumstances, and even a violation of the rights of citizens. For example, some believe it is unfair when random assignment results in some delinquents being sentenced to boot camp while others get assigned to a control group without any sanction at all or a less restrictive sanction than boot camp. In the MDVE, some believe it is unfair that some suspects were arrested and received an official record whereas others were not arrested for the same type of behavior. In other cases, subjects in the experimental group may receive some benefit from the treatment that is essentially denied to the control group for a period of time and this can become an issue as well.

There are other important reasons why random assignment is difficult to accomplish. Random assignment may, for example, involve a disruption of the normal procedures of agencies and their officers. In the MDVE, officers had to adjust their normal and established routine, and this was a barrier at times in that study. Shadish, Cook, and Campbell also note that random assignment may not always be feasible or desirable when quick answers are needed. 22 This is because experimental designs sometimes take a long time to produce results. In addition to the time required in planning and organizing the experiment, and treatment delivery, researchers may need several months if not years to collect and analyze the data before they have answers. This is particularly important because time is often of the essence in criminal justice research, especially in research efforts testing the effect of some policy or program where it is not feasible to wait years for answers. Waiting for the results of an experimental design means that many policy-makers may make decisions without the results.

Quasi-Experimental Designs

In general terms, quasi-experiments include a group of designs that lack random assignment. Quasi-experiments may also lack other parts, such as a pre-test or a control group, just like some experimental designs. The absence of random assignment, however, is the ingredient that transforms an otherwise experimental design into a quasi-experiment. Lacking random assignment is a major disadvantage because it increases the chances that the experimental and control groups differ on relevant factors before the treatment�both known and unknown�differences that may then emerge as alternative explanations of the outcomes.

Just like experimental designs, quasi-experimental designs can be organized in many different ways. This section will discuss three types of quasi-experiments: nonequivalent group design, one-group longitudinal design, and two-group longitudinal design.

Nonequivalent Group Design

The nonequivalent group design is perhaps the most common type of quasi-experiment. 23 Notice that it is very similar to the classic experimental design with the exception that it lacks random assignment (see Table 5.4). Additionally, what was labeled the experimental group in an experimental design is sometimes called the treatment group in the nonequivalent group design. What was labeled the control group in the experimental design is sometimes called the comparison group in the nonequivalent group design. This terminological distinction is an indicator that the groups were not created through random assignment.

TABLE 5.4 | Nonequivalent Group Design

NR = Not Randomly assigned

One of the main problems with the nonequivalent group design is that it lacks random assignment, and without random assignment, there is a greater chance that the treatment and comparison groups may be different in some way that can impact study results. Take, for example, a nonequivalent group design where a researcher is interested in whether an aggression-reduction treatment program can reduce inmate-on-inmate assaults in a prison setting. Assume that the researcher asked for inmates who had previously been involved in assaultive activity to volunteer for the aggression-reduction program. Suppose the researcher placed the first 50 volunteers into the treatment group and the next 50 volunteers into the comparison group. Note that this method of assignment is not random but rather first come, first serve.

Because the study utilized volunteers and there was no random assignment, it is possible that the first 50 volunteers placed into the treatment group differed significantly from the last 50 volunteers who were placed in the comparison group. This can lead to alternative explanations for the results. For example, if the treatment group was much younger than the comparison group, the researcher may find at the end of the program that the treatment group still maintained a higher rate of infractions than the comparison group�even after the aggression-reduction program! The conclusion might be that the aggression program actually increased the level of violence among the treatment group. This conclusion would likely be spurious and may be due to the age differential between the treatment and comparison groups. Indeed, research has revealed that younger inmates are significantly more likely to engage in prison assaults than older inmates. The fact that the treatment group incurred more assaults than the comparison group after the aggression-reduction program may only relate to the age differential between the groups, not that the program had no effect or that it somehow may have increased aggression. The previous example highlights the importance of random assignment and the potential problems that can occur in its absence.

Although researchers who utilize a quasi-experimental design are not able to randomly assign their subjects to groups, they can employ other techniques in an attempt to make the groups as equivalent as possible on known or measured factors before the treatment is given. In the example above, it is likely that the researcher would have known the age of inmates, their prior assault record, and various other pieces of information (e.g., previous prison stays). Through a technique called matching, the researcher could make sure the treatment and comparison groups were �matched� on these important factors before administering the aggression reduction program to the treatment group. This type of matching can be done individual to individual (e.g., subject #1 in treatment group is matched to a selected subject #1 in comparison group on age, previous arrests, gender), or aggregately, such that the comparison group is similar to the treatment group overall (e.g., average ages between groups are similar, equal proportions of males and females). Knowledge of these and other important variables, for example, would allow the researcher to make sure that the treatment group did not have heavy concentrations of younger or more frequent or serious offenders than the comparison group�factors that are related to assaultive activity independent of the treatment program. In short, matching allows the researcher some control over who goes into the treatment and comparison groups so as to balance these groups on important factors absent random assignment. If unbalanced on one or more factors, these factors could emerge as alternative explanations of the results. Figure 5.3 demonstrates the logic of matching both at the individual and aggregate level in a quasi-experimental design.

Matching is an important part of the nonequivalent group design. By matching, the researcher can approximate equivalence between the groups on important variables that may influence the post-test. However, it is important to note that a researcher can only match subjects on factors that they have information about�a researcher cannot match the treatment and comparison group members on factors that are unmeasured or otherwise unknown but which may still impact outcomes. For example, if the researcher has no knowledge about the number of previous incarcerations, the researcher cannot match the treatment and comparison groups on this factor. Matching also requires that the information used for matching is valid and reliable, which is not always the case. Agency records, for example, are notorious for inconsistencies, errors, omissions, and for being dated, but are often utilized for matching purposes. Asking survey questions to generate information for matching (for example, how many times have you been incarcerated?) can also be problematic because some respondents may lie, forget, or exaggerate their behavior or experiences.

In addition to the above considerations, the more factors a researcher wishes to match the group members on, the more difficult it becomes to find appropriate matches. Matching on prior arrests or age is less complex than matching on several additional pieces of information. Finally, matching is never considered superior to random assignment when the goal is to construct equitable groups. This is because there is a much higher likelihood of equivalence with random assignment on factors that are both measured and unknown to the researcher. Thus, the results produced from a nonequivalent group design, even with matching, are at a greater risk of alternative explanations than an experimental design that features random assignment.

FIGURE 5.3 | (a) Individual Matching (b) Aggregate Matching

The previous discussion is not to suggest that the nonequivalent group design cannot be useful in answering important research questions. Rather, it is to suggest that the nonequivalent group design, and hence any quasi-experiment, is more susceptible to alternative explanations than the classic experimental design because of the absence of random assignment. As a result, a researcher must be prepared to rule out potential alternative explanations. Quasi-experimental designs that lack a pre-test or a comparison group are even less desirable than the nonequivalent group design and are subject to additional alternative explanations because of these missing parts. Although the quasi-experiment may be all that is available and still can serve as an important design in evaluating the impact of a particular treatment, it is not preferable to the classic experiment. Researchers (and consumers) must be attuned to the potential issues of this design so as to make informed conclusions about the results produced from such research studies.

The Effects of Red Light Camera (RLC) Enforcement

On March 15, 2009, an article appeared in the Santa Cruz Sentinel entitled �Ticket�s in the Mail: Red-Light Cameras Questioned.� The article stated �while studies show fewer T-bone crashes at lights with cameras and fewer drivers running red lights, the number of rear-end crashes increases.� 24 The study mentioned in the newspaper, which showed fewer drivers running red lights with cameras, was conducted by Richard Retting, Susan Ferguson, and Charles Farmer of the Insurance Institute for Highway Safety (IIHS). 25 They completed a quasi-experimental study in Philadelphia to determine the impact of red light cameras (RLC) on red light violations. In the study, the researchers selected nine intersections�six of which were experimental sites that utilized RLCs and three comparison sites that did not utilize RLCs. The six experimental sites were located in Philadelphia, Pennsylvania, and the three comparison sites were located in Atlantic County, New Jersey. The researchers chose the comparison sites based on the proximity to Philadelphia, the ability to collect data using the same methods as at experimental intersections (e.g., the use of cameras for viewing red light traffic), and the fact that police officials in Atlantic County had offered assistance selecting and monitoring the intersections.

The authors collected three phases of information in the RLC study at the experimental and comparison sites:

Phase 1 Data Collection: Baseline (pre-test) data collection at the experimental and comparison sites consisting of the number of vehicles passing through each intersection, the number of red light violations, and the rate of red light violations per 10,000 vehicles.

Phase 2 Data Collection: Number of vehicles traveling through experimental and comparison intersections, number of red light violations after a 1-second yellow light increase at the experimental sites (treatment 1), number of red light violations at comparison sites without a 1-second yellow light increase, and red light violations per 10,000 vehicles at both experimental and comparison sites.

Phase 3 Data Collection: Red light violations after a 1-second yellow light increase and RLC enforcement at the experimental sites (treatment 2), red light violations at comparison sites without a 1-second yellow increase or RLC enforcement, number of vehicles passing through the experimental and comparison intersections, and the rate of red light violations per 10,000 vehicles.

The researchers operationalized �red light violations� as those where the vehicle entered the intersection one-half of a second or more after the onset of the red signal where the vehicle�s rear tires had to be positioned behind the crosswalk or stop line prior to entering on red. Vehicles already in the intersection at the onset of the red light, or those making a right turn on red with or without stopping were not considered red light violations.

The researchers collected video data at each of the experimental and comparison sites during Phases 1�3. This allowed the researchers to examine red light violations before, during, and after the implementation of red light enforcement and yellow light time increases. Based on an analysis of data, the researchers revealed that the implementation of a 1-second yellow light increase led to reductions in the rate of red light violations from Phase 1 to Phase 2 in all of the experimental sites. In 2 out of 3 comparison sites, the rate of red light violations also decreased, despite no yellow light increase. From Phase 2 to Phase 3 (the enforcement of red light camera violations in addition to a 1-second yellow light increase at experimental sites), the authors noted decreases in the rate of red light violations in all experimental sites, and decreases among 2 of 3 comparison sites without red light enforcement in effect.

Concluding their study, the researchers noted that the study �found large and highly significant incremental reductions in red light running associated with increased yellow signal timing followed by the introduction of red light cameras.� Despite these findings, the researchers noted a number of potential factors to consider in light of the findings: the follow-up time periods utilized when counting red light violations before and after the treatment conditions were instituted; publicity about red light camera enforcement; and the size of fines associated with red light camera enforcement (the fine in Philadelphia was $100, higher than in many other cities), among others.

After reading about the study used in the newspaper article, has your impression of the newspaper headline and quote changed?

For more information and research on the effect of RLCs, visit the Insurance Institute for Highway Safety at http://www .iihs.org/research/topics/rlr.html .

One-Group Longitudinal Design

Like all experimental designs, the quasi-experimental design can come in a variety of forms. The second quasi-experimental design (above) is the one-group longitudinal design (also called a simple interrupted time series design). 26 An examination of this design shows that it lacks both random assignment and a comparison group (see Table 5.5). A major difference between this design and others we have covered is that it includes multiple pre-test and post-test observations.

TABLE 5.5 | One-Group Longitudinal Design

The one-group longitudinal design is useful when researchers are interested in exploring longer-term patterns. Indeed, the term longitudinal generally means �over time��repeated measurements of the pre-test and post-test over time. This is different from cross-sectional designs, which examine the pre-test and post-test at only one point in time (e.g., at a single point before the application of the treatment and at a single point after the treatment). For example, in the nonequivalent group design and the classic experimental design previously examined, both are cross-sectional because pre-tests and post-tests are measured at one point in time (e.g., at a point 6 months after the treatment). Yet, these designs could easily be considered longitudinal if researchers took repeated measures of the pre-test and post-test.

The organization of the one-group longitudinal design is to examine a baseline of several pre-test observations, introduce a treatment or intervention, and then examine the post-test at several different time intervals. As organized, this design is useful for gauging the impact that a particular program, policy, or law has, if any, and how long the treatment impact lasts. Consider an example whereby a researcher is interested in gauging the impact of a tobacco ban on inmate-on-inmate assaults in a prison setting. This is an important question, for recent years have witnessed correctional systems banning all tobacco products from prison facilities. Correctional administrators predicted that there would be a major increase of inmate-on-inmate violence once the bans took effect. The one-group longitudinal design would be one appropriate design to examine the impact of banning tobacco on inmate assaults.

To construct this study using the one-group longitudinal design, the researcher would first examine the rate of inmate-on-inmate assaults in the prison system (or at an individual prison, a particular cellblock, or whatever the unit of analysis) prior to the removal of tobacco. This is the pre-test, or a baseline of assault activity before the ban goes into effect. In the design presented above, perhaps the researcher would measure the level of assaults in the preceding four months prior to the tobacco ban. When establishing a pre-test baseline, the general rule is that, in a longitudinal design, the more time utilized, both in overall time and number of intervals, the better. For example, the rate of assaults in the preceding month is not as useful as an entire year of data on inmate assaults prior to the tobacco ban. Next, once the tobacco ban is implemented, the researcher would then measure the rate of inmate assaults in the coming months to determine what impact the ban had on inmate-on-inmate assaults. This is shown in Table 5.5 as the multiple post-test measures of assaults. Assaults may increase, decrease, or remain constant from the pre-test baseline over the term of the post-test.

If assaults increased at the same time as the ban went into effect, the researcher might conclude that the increase was due only to the tobacco ban. But, could there be alternative explanations? The answer to this question is yes, there may be other plausible explanations for the increase even with several months of pre-test data. Unfortunately, without a comparison group there is no way for the researcher to be certain if the increase in assaults was due to the tobacco ban, or some other factor that may have spurred the increase in assaults and happened at the same time as the tobacco ban. What if assaults decreased after the tobacco ban went into effect? In this scenario, because there is no comparison group, the researcher would still not know if the results would have happened anyway without the tobacco ban. In these instances, the lack of a comparison group prevents the researcher from confidently attributing the results to the tobacco ban, and interpretation is subject to numerous alternative explanations.

Two-Group Longitudinal Design

A remedy for the previous situation would be to introduce a comparison group (see Table 5.6). Prior to the full tobacco ban, suppose prison administrators conducted a pilot program at one prison to provide insight as to what would happen once the tobacco ban went into effect systemwide. To conduct this pilot, the researcher identified one prison. At this prison, the researcher identified two different cellblocks, C-Block and D-Block. C-Block constitutes the treatment group, or the cellblock of inmates who will have their tobacco taken away. D-Block is the comparison group�inmates in this cellblock will retain their tobacco privileges during the course of the study and during a determined follow-up period to measure post-test assaults (e.g., 12-months). This is a two-group longitudinal design (also sometimes called a multiple interrupted time series design), and adding a comparison group makes this design superior to the one-group longitudinal design.

TABLE 5.6 | Two-Group Longitudinal Design

The usefulness of adding a comparison group to the study means that the researcher can have more confidence that the results at the post-test are due to the tobacco ban and not some alternative explanation. This is because any difference in assaults at the post-test between the treatment and comparison group should be attributed to the only difference between them, the tobacco ban. For this interpretation to hold, however, the researcher must be sure that C-Block and D-Block are similar or equivalent on all factors that might influence the post-test. There are many potential factors that should be considered. For example, the researcher will want to make sure that the same types of inmates are housed in both cellblocks. If a chronic group of assaultive inmates constitutes members of C-Block, but not D-Block, this differential could explain the results, not the treatment.

The researcher might also want to make sure equitable numbers of tobacco and non-tobacco users are found in each cellblock. If very few inmates in C-Block are smokers, the real effect of removing tobacco may be hidden. The researcher might also examine other areas where potential differences might arise, for example, that both cellblocks are staffed with equal numbers of officers, that officers in each cellblock tend to resolve inmate disputes similarly, and other potential issues that could influence post-test measure of assaults. Equivalence could also be ensured by comparing the groups on additional evidence before the ban takes effect: number of prior prison sentences, time served in prison, age, seriousness of conviction crime, and other factors that might relate to assaultive behavior, regardless of the tobacco ban. Moreover, the researcher should ensure that inmates in C-Block do not know that their D-Block counterparts are still allowed tobacco during the pilot study, and vice versa. If either group knows about the pilot program being an experiment, they might act differently than normal, and this could become an explanation of results. Additionally, the researchers might also try to make sure that C-Block inmates are completely tobacco free after the ban goes into effect�that they do not hoard, smuggle, or receive tobacco from officers or other inmates during the tobacco ban in or outside of the cellblock. If these and other important differences are accounted for at the individual and cellblock level, the researcher will have more confidence that any differences in assaults at the post-test between the treatment and comparison groups are related to the tobacco ban, and not some other difference between the two groups or the two cellblocks.

The addition of a comparison group aids in the ability of the researcher to isolate the true impact of a tobacco ban on inmate-on-inmate assaults. All factors that influence the treatment group should also influence the comparison group because the groups are made up of equivalent individuals in equivalent circumstances, with the exception of the tobacco ban. If this is the only difference, the results can be attributed to the ban. Although the addition of the comparison group in the two-group longitudinal design provides more confidence that the findings are attributed to the tobacco ban, the fact that this design lacks randomization means that alternative explanations cannot be completely ruled out�but they can be minimized. This example also suggests that the quasi-experiment in this instance may actually be preferable to an experimental design�noting the realities of prison administration. For example, prison inmates are not typically randomly assigned to different cellblocks by prison officers. Moreover, it is highly unlikely that a prison would have two open cellblocks waiting for a researcher to randomly assign incoming inmates to the prison for a tobacco ban study. Therefore, it is likely there would be differences among the groups in the quasi-experiment.

Fortunately, if differences between the groups are present, the researcher can attempt to determine their potential impact before interpretation of results. The researcher can also use statistical models after the ban takes effect to determine the impact of any differences between the groups on the post-test. While the two-group longitudinal quasi-experiment just discussed could also take the form of an experimental design, if random assignment could somehow be accomplished, the previous discussion provides one situation where an experimental design might be appropriate and desired for a particular research question, but would not be realistic considering the many barriers.

The Threat of Alternative Explanations

Alternative explanations are those factors that could explain the post-test results, other than the treatment. Throughout this chapter, we have noted the potential for alternative explanations and have given several examples of explanations other than the treatment. It is important to know that potential alternative explanations can arise in any research design discussed in this chapter. However, alternative explanations often arise because some design part is missing, for example, random assignment, a pre-test, or a control or comparison group. This is especially true in criminal justice where researchers often conduct field studies and have less control over their study conditions than do researchers who conduct experiments under highly controlled laboratory conditions. A prime example of this is the tobacco ban study, where it would be difficult for researchers to ensure that C-Block inmates, the treatment group, were completely tobacco free during the course of the study.

Alternative explanations are typically referred to as threats to internal validity. In this context, if an experiment is internally valid, it means that alternative explanations have been ruled out and the treatment is the only factor that produced the results. If a study is not internally valid, this means that alternative explanations for the results exist or potentially exist. In this section, we focus on some common alternative explanations that may arise in experimental and quasi-experimental designs. 27

Selection Bias

One of the more common alternative explanations that may occur is selection bias. Selection bias generally indicates that the treatment group (or experimental group) is somehow different from the comparison group (or control group) on a factor that could influence the post-test results. Selection bias is more often a threat in quasi-experimental designs than experimental designs due to the lack of random assignment. Suppose in our study of the prison tobacco ban, members of C-Block were substantially younger than members of D-Block, the comparison group. Such an imbalance between the groups would mean the researcher would not know if the differences in assaults are real (meaning the result of the tobacco ban) or a result of the age differential. Recall that research shows that younger inmates are more assaultive than older inmates and so we would expect more assaults among the younger offenders independent of the tobacco ban.

In a quasi-experiment, selection bias is perhaps the most prevalent type of alternative explanation and can seriously compromise results. Indeed, many of the examples above have referred to potential situations where the groups are imbalanced or not equivalent on some important factor. Although selection bias is a common threat in quasi-experimental designs because of lack of random assignment, and can be a threat in experimental designs because the groups could differ by chance alone or the practice of randomization was not maintained throughout the study (see Classics in CJ Research-MDVE above), a researcher may be able to detect such differentials. For example, the researcher could detect such differences by comparing the groups on the pre-test or other types of information before the start of the study. If differences were found, the researcher could take measures to correct them. The researcher could also use a statistical model that could account or control for differences between the groups and isolate the impact of the treatment, if any. This discussion is beyond the scope of this text but would be a potential way to deal with selection bias and estimate the impact of this bias on study results. The researcher could also, if possible, attempt to re-match the groups in a quasi-experiment or randomly assign the groups a second time in an experimental design to ensure equivalence. At the least, the researcher could recognize the group differences and discuss their potential impact on the results. Without a pre-test or other pre-study information on study participants, however, such differences might not be able to be detected and, therefore, it would be more difficult to determine how the differences, as a result of selection bias, influenced the results.

Another potential alternative explanation is history. History refers to any event experienced differently by the treatment and comparison groups in the time between the pre-test and the post-test that could impact results. Suppose during the course of the tobacco ban study several riots occurred on D-Block, the comparison group. Because of the riots, prison officers �locked down� this cellblock numerous times. Because D-Block inmates were locked down at various times, this could have affected their ability to otherwise engage in inmate assaults. At the end of the study, the assaults in D-Block might have decreased from their pre-test levels because of the lockdowns, whereas in C-Block assaults may have occurred at their normal pace because there was not a lockdown, or perhaps even increased from the pretest because tobacco was also taken away. Even if the tobacco ban had no effect and assaults remained constant in C-Block from pre- to post-test, the lockdown in D-Block might make it appear that the tobacco ban led to increased assaults in C-Block. Thus, the researcher would not know if the post-test results for the C-Block treatment group were attributable to the tobacco ban or the simple fact that D-Block inmates were locked down and their assault activity was artificially reduced. In this instance, the comparison group becomes much less useful because the lockdown created a historical factor that imbalanced the groups during the treatment phase and nullified the comparison.

Another potential alternative explanation is maturation. Maturation refers to the natural biological, psychological, or emotional processes we all experience as time passes�aging, becoming more or less intelligent, becoming bored, and so on. For example, if a researcher was interested in the effect of a boot camp on recidivism for juvenile offenders, it is possible that over the course of the boot camp program the delinquents naturally matured as they aged and this produced the reduction in recidivism�not that the boot camp somehow led to this reduction. This threat is particularly applicable in situations that deal with populations that rapidly change over a relatively short period of time or when a treatment lasts a considerable period of time. However, this threat could be eliminated with a comparison group that is similar to the treatment group. This is because the maturation effects would occur in both groups and the effect of the boot camp, if any, could be isolated. This assumes, however, that the groups are matched and equitable on factors subject to the maturation process, such as age. If not, such differentials could be an alternative explanation of results. For example, if the treatment and comparison groups differ by age, on average, this could mean that one group changes or matures at a different rate than the other group. This differential rate of change or maturation as a result of the age differential could explain the results, not the treatment. This example demonstrates how selection bias and maturation can interact at the same time as alternative explanations. This example also suggests the importance of an equivalent control or comparison group to eliminate or minimize the impact of maturation as an alternative explanation.

Attrition or Subject Mortality

Attrition or subject mortality is another typical alternative explanation. Attrition refers to differential loss in the number or type of subjects between the treatment and comparison groups and can occur in both experimental and quasi-experimental designs. Suppose we wanted to conduct a study to determine who is the better research methods professor among the authors of this textbook. Let�s assume that we have an experimental design where students were randomly assigned to professor 1, professor 2, or professor 3. By randomly assigning students to each respective professor, there is greater probability that the groups are equivalent and thus there are no differences between the three groups with one exception�the professor they receive and his or her particular teaching and delivery style. This is the treatment. Let�s also assume that the professors will be administering the same tests and using the same textbook. After the group members are randomly assigned, a pre-treatment evaluation shows the groups are in fact equivalent on all important known factors that could influence post-test scores, such as grade point average, age, time in school, and exposure to research methods concepts. Additionally, all groups scored comparably on a pre-test of knowledge about research methods, thus there is more confidence that the groups are in fact equivalent.

At the conclusion of the study, we find that professor 2�s group has the lowest final test scores of the three. However, because professor 2 is such an outstanding professor, the results appear odd. At first glance, the researcher thinks the results could have been influenced by students dropping out of the class. For example, perhaps several of professor 2�s students dropped the course but none did from the classes of professor 1 or 3. It is revealed, however, that an equal number of students dropped out of all three courses before the post-test and, therefore, this could not be the reason for the low scores in professor 2�s course. Upon further investigation, however, the researcher finds that although an equal number of students dropped out of each class, the dropouts in professor 2�s class were some of his best students. In contrast, those who dropped out of professor 1�s and professor 3�s courses were some of their poorest students. In this example, professor 2 appears to be the least effective teacher. However, this result appears to be due to the fact that his best students dropped out, and this highly influenced the final test average for his group. Although there was not a differential loss of subjects in terms of numbers (which can also be an attrition issue), there was differential loss in the types of students. This differential loss, not the teaching style, is an alternative explanation of the results.

Testing or Testing Bias

Another potential alternative explanation is testing or testing bias. Suppose that after the pre-test of research methods knowledge, professor 1 and professor 3 reviewed the test with their students and gave them the correct answers. Professor 2 did not. The fact that professor l�s and professor 3�s groups did better on the post-test final exam may be explained by the finding that students in those groups remembered the answers to the pre-test, were thus biased at the pre-test, and this artificially inflated their post-test scores. Testing bias can explain the results because students in groups 1 and 3 may have simply remembered the answers from the pre-test review. In fact, the students in professor l�s and 3�s courses may have scored high on the post-test without ever having been exposed to the treatment because they were biased at the pre-test.

Instrumentation

Another alternative explanation that can arise is instrumentation. Instrumentation refers to changes in the measuring instrument from pre- to post-test. Using the previous example, suppose professors 1 and 3 did not give the same final exam as professor 2. For example, professors 1 and 3 changed the final exam and professor 2 kept the final exam the same as the pretest. Because professors 1 and 3 changed the exam, and perhaps made it easier or somehow different from the pre-test exam, results that showed lower scores for professor 2�s students may be related only to instrumentation changes from pre- to post-test. Obviously, to limit the influence of instrumentation, researchers should make sure that instruments remain consistent from pre- to post-test.

A final alternative explanation is reactivity. Reactivity occurs when members of the treatment or experimental group change their behavior simply as a result of being part of a study. This is akin to the finding that people tend to change their behavior when they are being watched or are aware they are being studied. If members of the experiment know they are part of an experiment and are being studied and watched, it is possible that their behavior will change independent of the treatment. If this occurs, the researcher will not know if the behavior change is the result of the treatment, or simply a result of being part of a study. For example, suppose a researcher wants to determine if a boot camp program impacts the recidivism of delinquent offenders. Members of the experimental group are sentenced to boot camp and members of the control group are released on their own recognizance to their parents. Because members of the experimental group know they are part of the experiment, and hence being watched closely after they exit boot camp, they may artificially change their behavior and avoid trouble. Their change of behavior may be totally unrelated to boot camp, but rather, to their knowledge of being part of an experiment.

Other Potential Alternative Explanations

The above discussion provided some typical alternative explanations that may arise with the designs discussed in this chapter. There are, however, other potential alternative explanations that may arise. These alternative explanations arise only when a control or comparison group is present.

One such alternative explanation is diffusion of treatment. Diffusion of treatment occurs when the control or comparison group learns about the treatment its members are being denied and attempts to mimic the behavior of the treatment group. If the control group is successful in mimicking the experimental group, for example, the results at the end of the study may show similarity in outcomes between groups and cause the researcher to conclude that the program had no effect. In fact, however, the finding of no effect can be explained by the comparison group mimicking the treatment group. 28 In reality, there may be no effect of the treatment, but the researcher would not know this for sure because the control group effectively transformed into another experimental group�there is then no baseline of comparison. Consider a study where a researcher wants to determine the impact of a training program on class behavior and participation. In this study, the experimental group is exposed to several sessions of training on how to act appropriately in class and how to engage in class participation. The control group does not receive such training, but they are aware that they are part of an experiment. Suppose after a few class sessions the control group starts to mimic the behavior of the experimental group, acting the same way and participating in class the same way. At the conclusion of the study, the researcher might determine that the program had no impact because the comparison group, which did not receive the new program, showed similar progress.

In a related explanation, sometimes the comparison or control group learns about the experiment and attempts to compete with the experimental or treatment group. This alternative explanation is called compensatory rivalry. For example, suppose a police chief wants to determine if a new training program will increase the endurance of SWAT team officers. The chief randomly assigns SWAT members to either an experimental or control group. The experimental group will receive the new endurance training program and the control group will receive the normal program that has been used for years. During the course of the study, suppose the control group learns that the treatment group is receiving the new endurance program and starts to compete with the experimental group. Perhaps the control group runs five more miles per day and works out an extra hour in the weight room, in addition to their normal endurance program. At the end of the study, and due to the control group�s extra and competing effort, the results might show no effect of the new endurance program, and at worst, experimental group members may show a decline in endurance compared to the control group. The rivalry or competing behavior actually explains the results, not that the new endurance program has no effect or a damaging effect. Although the new endurance program may in reality have no effect, this cannot be known because of the actions of the control group, who learned about the treatment and competed with the experimental group.

Closely related to compensatory rivalry is the alternative explanation of comparison or control group demoralization. 29 In this instance, instead of competing with the experimental or treatment group, the control or comparison group simply gives up and changes their normal behavior. Using the SWAT example, perhaps the control group simply quits their normal endurance program when they learn about the treatment group receiving the new endurance program. At the post-test, their endurance will likely drop considerably compared to the treatment group. Because of this, the new endurance program might emerge as a shining success. In reality, however, the researcher will not know if any changes in endurance between the experimental and control groups are a result of the new endurance program or the control group giving up. Due to their giving up, there is no longer a comparison group of equitable others, the change in endurance among the treatment group members could be attributed to a number of alternative explanations, for example, maturation. If the comparison group behaves normally, the researcher will be able to exclude maturation as a potential explanation. This is because any maturation effects will occur in both groups.

The previous discussion suggests that when the control or comparison group learns about the experiment and the treatment they are denied, potential alternative explanations can arise. Perhaps the best remedy to protect from the alternative explanations just discussed is to make sure the treatment and comparison groups do not have contact with one another. In laboratory experiments this can be ensured, but sometimes this is a problem in criminal justice studies, which are often conducted in the field.

The previous discussion also suggests that there are numerous alternative explanations that can impact the interpretation of results from a study. A careful researcher would know that alternative explanations must be ruled out before reaching a definitive conclusion about the impact of a particular program. The researcher must be attuned to these potential alternative explanations because they can influence results and how results are interpreted. Moreover, the discussion shows that several alternative explanations can occur at the same time. For example, it is possible that selection bias, maturation, attrition, and compensatory rivalry all emerge as alternative explanations in the same study. Knowing about these potential alternative explanations and how they can impact the results of a study is what distinguishes a consumer of research from an educated consumer of research.

Chapter Summary

The primary focus of this chapter was the classic experimental design, the foundation for other types of experimental and quasi-experimental designs. The classic experimental design is perhaps the most useful design when exploring causal relationships. Often, however, researchers cannot employ the classic experimental design to answer a research question. In fact, the classic experimental design is rare in criminal justice and criminology because it is often difficult to ensure random assignment for a variety of reasons. In circumstances where an experimental design is appropriate but not feasible, researchers may turn to one of many quasi-experimental designs. The most important difference between the two is that quasi-experimental designs do not feature random assignment. This can create potential problems for researchers. The main problem is that there is a greater chance the treatment and comparison groups may differ on important characteristics that could influence the results of a study. Although researchers can attempt to prevent imbalances between the groups by matching them on important known characteristics, it is still much more difficult to establish equivalence than it is in the classic experiment. As such, it becomes more difficult to determine what impact a treatment had, if any, as one moves from an experimental to a quasi-experimental design.

Perhaps the most important lesson to be learned in this chapter is that to be an educated consumer of research results requires an understanding of the type of design that produced the results. There are numerous ways experimental and quasi-experimental designs can be structured. This is why much attention was paid to the classic experimental design. In reality, all experimental and quasi-experimental designs are variations of the classic experiment in some way�adding or deleting certain components. If the components and organization and logic of the classic experimental design are understood, consumers of research will have a better understanding of the results produced from any sort of research design. For example, what problems in interpretation arise when a design lacks a pre-test, a control group, or random assignment? Having an answer to this question is a good start toward being an informed consumer of research results produced through experimental and quasi-experimental designs.

Critical Thinking Questions

1. Why is randomization/random assignment preferable to matching? Provide several reasons with explanation.

2. What are some potential reasons a researcher would not be able to utilize random assignment?

3. What is a major limitation of matching?

4. What is the difference between a longitudinal study and a cross-sectional study?

5. Describe a hypothetical study where maturation, and not the treatment, could explain the outcomes of the research.

association (or covariance or correlation): One of three conditions that must be met for establishing cause and effect, or a causal relationship. Association refers to the condition that X and Y must be related for a causal relationship to exist. Association is also referred to as covariance or correlation. Although two variables may be associated (or covary or be correlated), this does not automatically imply that they are causally related

attrition or subject mortality: A threat to internal validity, it refers to the differential loss of subjects between the experimental (treatment) and control (comparison) groups during the course of a study

cause and effect relationship: A cause and effect relationship occurs when one variable causes another, and no other explanation for that relationship exists

classic experimental design or experimental design: A design in a research study that features random assignment to an experimental or control group. Experimental designs can vary tremendously, but a constant feature is random assignment, experimental and control groups, and a post-test. For example, a classic experimental design features random assignment, a treatment, experimental and control groups, and pre- and post-tests

comparison group: The group in a quasi-experimental design that does not receive the treatment. In an experimental design, the comparison group is referred to as the control group

compensatory rivalry: A threat to internal validity, it occurs when the control or comparison group attempts to compete with the experimental or treatment group

control group: In an experimental design, the control group does not receive the treatment. The control group serves as a baseline of comparison to the experimental group. It serves as an example of what happens when a group equivalent to the experimental group does not receive the treatment

cross-sectional designs: A measurement of the pre-test and post-test at one point in time (e.g., six months before and six months after the program)

demoralization: A threat to internal validity closely associated with compensatory rivalry, it occurs when the control or comparison group gives up and changes their normal behavior. While in compensatory rivalry the group members compete, in demoralization, they simply quit. Both are not normal behavioral reactions

dependent variable: Also known as the outcome in a research study. A post-test is a measure of the dependent variable

diffusion of treatment: A threat to internal validity, it occurs when the control or comparison group members learn that they are not getting the treatment and attempt to mimic the behavior of the experimental or treatment group. This mimicking may make it seem as if the treatment is having no effect, when in fact it may be

elimination of alternative explanations: One of three conditions that must be met for establishing cause and effect. Elimination of alternative explanations means that the researcher has ruled out other explanations for an observed relationship between X and Y

experimental group: In an experimental design, the experimental group receives the treatment

history: A threat to internal validity, it refers to any event experienced differently by the treatment and comparison groups�an event that could explain the results other than the supposed cause

independent variable: Also called the cause

instrumentation: A threat to internal validity, it refers to changes in the measuring instrument from pre- to post-test

longitudinal: Refers to repeated measurements of the pre-test and post-test over time, typically for the same group of individuals. This is the opposite of cross-sectional

matching: A process sometimes utilized in some quasi-experimental designs that feature treatment and comparison groups. Matching is a process whereby the researcher attempts to ensure equivalence between the treatment and comparison groups on known information, in the absence of the ability to randomly assign the groups

maturation: A threat to internal validity, maturation refers to the natural biological, psychological, or emotional processes as time passes

negative association: Refers to a negative association between two variables. A negative association is demonstrated when X increases and Y decreases, or X decreases and Y increases. Also known as an inverse relationship�the variables moving in opposite directions

operationalized or operationalization: Refers to the process of assigning a working definition to a concept. For example, the concept of intelligence can be operationalized or defined as grade point average or score on a standardized exam, among others

pilot program or test: Refers to a smaller test study or pilot to work out problems before a larger study and to anticipate changes needed for a larger study. Similar to a test run

positive association: Refers to a positive association between two variables. A positive association means as X increases, Y increases, or as X decreases, Y decreases

post-test: The post-test is a measure of the dependent variable after the treatment has been administered

pre-test: The pre-test is a measure of the dependent variable or outcome before a treatment is administered

quasi-experiment: A quasi-experiment refers to any number of research design configurations that resemble an experimental design but primarily lack random assignment. In the absence of random assignment, quasi-experimental designs feature matching to attempt equivalence

random assignment: Refers to a process whereby members of the experimental group and control group are assigned to each group through a random and unbiased process

random selection: Refers to selecting a smaller but representative subset from a population. Not to be confused with random assignment

reactivity: A threat to internal validity, it occurs when members of the experimental (treatment) or control (comparison) group change their behavior unnaturally as a result of being part of a study

selection bias: A threat to internal validity, selection bias occurs when the experimental (treatment) group and control (comparison) group are not equivalent. The difference between the groups can be a threat to internal validity, or, an alternative explanation to the findings

spurious: A spurious relationship is one where X and Y appear to be causally related, but in fact the relationship is actually explained by a variable or factor other than X

testing or testing bias: A threat to internal validity, it refers to the potential of study members being biased prior to a treatment, and this bias, rather than the treatment, may explain study results

threat to internal validity: Also known as alternative explanation to a relationship between X and Y. Threats to internal validity are factors that explain Y, or the dependent variable, and are not X, or the independent variable

timing: One of three conditions that must be met for establishing cause and effect. Timing refers to the condition that X must come before Y in time for X to be a cause of Y. While timing is necessary for a causal relationship, it is not sufficient, and considerations of association and eliminating other alternative explanations must be met

treatment: A component of a research design, it is typically denoted by the letter X. In a research study on the impact of teen court on juvenile recidivism, teen court is the treatment. In a classic experimental design, the treatment is given only to the experimental group, not the control group

treatment group: The group in a quasi-experimental design that receives the treatment. In an experimental design, this group is called the experimental group

unit of analysis: Refers to the focus of a research study as being individuals, groups, or other units of analysis, such as prisons or police agencies, and so on

variable(s): A variable is a concept that has been given a working definition and can take on different values. For example, intelligence can be defined as a person�s grade point average and can range from low to high or can be defined numerically by different values such as 3.5 or 4.0

1 Povitsky, W., N. Connell, D. Wilson, & D. Gottfredson. (2008). �An experimental evaluation of teen courts.� Journal of Experimental Criminology, 4, 137�163.

2 Hirschi, T., and H. Selvin (1966). �False criteria of causality in delinquency.� Social Problems, 13, 254�268.

3 Robert Roy Britt, �Churchgoers Live Longer.� April, 3, 2006. http://www.livescience.com/health/060403_church_ good.html. Retrieved on September 30, 2008.

4 Kalist, D., and D. Yee (2009). �First names and crime: Does unpopularity spell trouble?� Social Science Quarterly, 90 (1), 39�48.

5 Sherman, L. (1992). Policing domestic violence. New York: The Free Press.

6 For historical and interesting reading on the effects of weather on crime and other disorder, see Dexter, E. (1899). �Influence of weather upon crime.� Popular Science Monthly, 55, 653�660 in Horton, D. (2000). Pioneering Perspectives in Criminology. Incline Village, NV: Copperhouse.

7 http://www.escapistmagazine.com/news/view/111191-Less-Crime-in-U-S-Thanks-to-Videogames , retrieved on September 13, 2011. This news article was in response to a study titled �Understanding the effects of violent videogames on violent crime.� See Cunningham, Scott, Engelst�tter, Benjamin, and Ward, (April 7, 2011). Available at SSRN: http://ssm.com/abstract= 1804959.

8 Cohn, E. G. (1987). �Changing the domestic violence policies of urban police departments: Impact of the Minneapolis experiment.� Response, 10 (4), 22�24.

9 Schmidt, Janell D., & Lawrence W. Sherman (1993). �Does arrest deter domestic violence?� American Behavioral Scientist, 36 (5), 601�610.

10 Maxwell, Christopher D., Joel H. Gamer, & Jeffrey A. Fagan. (2001). The effects of arrest on intimate partner violence: New evidence for the spouse assault replication program. Washington D.C.: National Institute of Justice.

11 Miller, N. (2005). What does research and evaluation say about domestic violence laws? A compendium of justice system laws and related research assessments. Alexandria, VA: Institute for Law and Justice.

12 The sections on experimental and quasi-experimental designs rely heavily on the seminal work of Campbell and Stanley (Campbell, D.T., & J. C. Stanley. (1963). Experimental and quasi-experimental designs for research. Chicago: RandMcNally) and more recently, Shadish, W., T. Cook, & D. Campbell. (2002). Experimental and quasi-experimental designs for generalized causal inference. New York: Houghton Mifflin.

13 Povitsky et al. (2008). p. 146, note 9.

14 Shadish, W., T. Cook, & D. Campbell. (2002). Experimental and quasi-experimental designs for generalized causal inference. New York: Houghton Mifflin Company.

15 Ibid, 15.

16 Finckenauer, James O. (1982). Scared straight! and the panacea phenomenon. Englewood Cliffs, N.J.: Prentice Hall.

17 Yarborough, J.C. (1979). Evaluation of JOLT (Juvenile Offenders Learn Truth) as a deterrence program. Lansing, MI: Michigan Department of Corrections.

18 Petrosino, Anthony, Carolyn Turpin-Petrosino, & James O. Finckenauer. (2000). �Well-meaning programs can have harmful effects! Lessons from experiments of programs such as Scared Straight.� Crime and Delinquency, 46, 354�379.

19 �Swearing makes pain more tolerable� retrieved at http:// www.livescience.com/health/090712-swearing-pain.html (July 13, 2009). Also see �Bleep! My finger! Why swearing helps ease pain� by Tiffany Sharpies, retrieved at http://www.time.com/time/health/article /0,8599,1910691,00.html?xid=rss-health (July 16, 2009).

20 For an excellent discussion of the value of controlled experiments and why they are so rare in the social sciences, see Sherman, L. (1992). Policing domestic violence. New York: The Free Press, 55�74.

21 For discussion, see Weisburd, D., T. Einat, & M. Kowalski. (2008). �The miracle of the cells: An experimental study of interventions to increase payment of court-ordered financial obligations.� Criminology and Public Policy, 7, 9�36.

22 Shadish, Cook, & Campbell. (2002).

24 Kelly, Cathy. (March 15, 2009). �Tickets in the mail: Red-light cameras questioned.� Santa Cruz Sentinel.

25 Retting, Richard, Susan Ferguson, & Charles Farmer. (January 2007). �Reducing red light running through longer yellow signal timing and red light camera enforcement: Results of a field investigation.� Arlington, VA: Insurance Institute for Highway Safety.

26 Shadish, Cook, & Campbell. (2002).

27 See Shadish, Cook, & Campbell. (2002), pp. 54�61 for an excellent discussion of threats to internal validity. Also see Chapter 2 for an extended discussion of all forms of validity considered in research design.

28 Trochim, W. (2001). The research methods knowledge base, 2nd ed. Cincinnati, OH: Atomic Dog.

Applied Research Methods in Criminal Justice and Criminology by University of North Texas is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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The use and interpretation of quasi-experimental design

Last updated

6 February 2023

Reviewed by

Miroslav Damyanov

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• What is a quasi-experimental design?

Commonly used in medical informatics (a field that uses digital information to ensure better patient care), researchers generally use this design to evaluate the effectiveness of a treatment – perhaps a type of antibiotic or psychotherapy, or an educational or policy intervention.

Even though quasi-experimental design has been used for some time, relatively little is known about it. Read on to learn the ins and outs of this research design.

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• When to use a quasi-experimental design

A quasi-experimental design is used when it's not logistically feasible or ethical to conduct randomized, controlled trials. As its name suggests, a quasi-experimental design is almost a true experiment. However, researchers don't randomly select elements or participants in this type of research.

Researchers prefer to apply quasi-experimental design when there are ethical or practical concerns. Let's look at these two reasons more closely.

Ethical reasons

In some situations, the use of randomly assigned elements can be unethical. For instance, providing public healthcare to one group and withholding it to another in research is unethical. A quasi-experimental design would examine the relationship between these two groups to avoid physical danger.

Practical reasons

Randomized controlled trials may not be the best approach in research. For instance, it's impractical to trawl through large sample sizes of participants without using a particular attribute to guide your data collection .

Recruiting participants and properly designing a data-collection attribute to make the research a true experiment requires a lot of time and effort, and can be expensive if you don’t have a large funding stream.

A quasi-experimental design allows researchers to take advantage of previously collected data and use it in their study.

• Examples of quasi-experimental designs

Quasi-experimental research design is common in medical research, but any researcher can use it for research that raises practical and ethical concerns. Here are a few examples of quasi-experimental designs used by different researchers:

Example 1: Determining the effectiveness of math apps in supplementing math classes

A school wanted to supplement its math classes with a math app. To select the best app, the school decided to conduct demo tests on two apps before selecting the one they will purchase.

Scope of the research

Since every grade had two math teachers, each teacher used one of the two apps for three months. They then gave the students the same math exams and compared the results to determine which app was most effective.

Reasons why this is a quasi-experimental study

This simple study is a quasi-experiment since the school didn't randomly assign its students to the applications. They used a pre-existing class structure to conduct the study since it was impractical to randomly assign the students to each app.

Example 2: Determining the effectiveness of teaching modern leadership techniques in start-up businesses

A hypothetical quasi-experimental study was conducted in an economically developing country in a mid-sized city.

Five start-ups in the textile industry and five in the tech industry participated in the study. The leaders attended a six-week workshop on leadership style, team management, and employee motivation.

After a year, the researchers assessed the performance of each start-up company to determine growth. The results indicated that the tech start-ups were further along in their growth than the textile companies.

The basis of quasi-experimental research is a non-randomized subject-selection process. This study didn't use specific aspects to determine which start-up companies should participate. Therefore, the results may seem straightforward, but several aspects may determine the growth of a specific company, apart from the variables used by the researchers.

Example 3: A study to determine the effects of policy reforms and of luring foreign investment on small businesses in two mid-size cities

In a study to determine the economic impact of government reforms in an economically developing country, the government decided to test whether creating reforms directed at small businesses or luring foreign investments would spur the most economic development.

The government selected two cities with similar population demographics and sizes. In one of the cities, they implemented specific policies that would directly impact small businesses, and in the other, they implemented policies to attract foreign investment.

After five years, they collected end-of-year economic growth data from both cities. They looked at elements like local GDP growth, unemployment rates, and housing sales.

The study used a non-randomized selection process to determine which city would participate in the research. Researchers left out certain variables that would play a crucial role in determining the growth of each city. They used pre-existing groups of people based on research conducted in each city, rather than random groups.

• Advantages of a quasi-experimental design

Some advantages of quasi-experimental designs are:

Researchers can manipulate variables to help them meet their study objectives.

It offers high external validity, making it suitable for real-world applications, specifically in social science experiments.

Integrating this methodology into other research designs is easier, especially in true experimental research. This cuts down on the time needed to determine your outcomes.

• Disadvantages of a quasi-experimental design

Despite the pros that come with a quasi-experimental design, there are several disadvantages associated with it, including the following:

It has a lower internal validity since researchers do not have full control over the comparison and intervention groups or between time periods because of differences in characteristics in people, places, or time involved. It may be challenging to determine whether all variables have been used or whether those used in the research impacted the results.

There is the risk of inaccurate data since the research design borrows information from other studies.

There is the possibility of bias since researchers select baseline elements and eligibility.

• What are the different quasi-experimental study designs?

There are three distinct types of quasi-experimental designs:

Nonequivalent

Regression discontinuity, natural experiment.

This is a hybrid of experimental and quasi-experimental methods and is used to leverage the best qualities of the two. Like the true experiment design, nonequivalent group design uses pre-existing groups believed to be comparable. However, it doesn't use randomization, the lack of which is a crucial element for quasi-experimental design.

Researchers usually ensure that no confounding variables impact them throughout the grouping process. This makes the groupings more comparable.

Example of a nonequivalent group design

A small study was conducted to determine whether after-school programs result in better grades. Researchers randomly selected two groups of students: one to implement the new program, the other not to. They then compared the results of the two groups.

This type of quasi-experimental research design calculates the impact of a specific treatment or intervention. It uses a criterion known as "cutoff" that assigns treatment according to eligibility.

Researchers often assign participants above the cutoff to the treatment group. This puts a negligible distinction between the two groups (treatment group and control group).

Example of regression discontinuity

Students must achieve a minimum score to be enrolled in specific US high schools. Since the cutoff score used to determine eligibility for enrollment is arbitrary, researchers can assume that the disparity between students who only just fail to achieve the cutoff point and those who barely pass is a small margin and is due to the difference in the schools that these students attend.

Researchers can then examine the long-term effects of these two groups of kids to determine the effect of attending certain schools. This information can be applied to increase the chances of students being enrolled in these high schools.

This research design is common in laboratory and field experiments where researchers control target subjects by assigning them to different groups. Researchers randomly assign subjects to a treatment group using nature or an external event or situation.

However, even with random assignment, this research design cannot be called a true experiment since nature aspects are observational. Researchers can also exploit these aspects despite having no control over the independent variables.

Example of the natural experiment approach

An example of a natural experiment is the 2008 Oregon Health Study.

Oregon intended to allow more low-income people to participate in Medicaid.

Since they couldn't afford to cover every person who qualified for the program, the state used a random lottery to allocate program slots.

Researchers assessed the program's effectiveness by assigning the selected subjects to a randomly assigned treatment group, while those that didn't win the lottery were considered the control group.

• Differences between quasi-experiments and true experiments

There are several differences between a quasi-experiment and a true experiment:

Participants in true experiments are randomly assigned to the treatment or control group, while participants in a quasi-experiment are not assigned randomly.

In a quasi-experimental design, the control and treatment groups differ in unknown or unknowable ways, apart from the experimental treatments that are carried out. Therefore, the researcher should try as much as possible to control these differences.

Quasi-experimental designs have several "competing hypotheses," which compete with experimental manipulation to explain the observed results.

Quasi-experiments tend to have lower internal validity (the degree of confidence in the research outcomes) than true experiments, but they may offer higher external validity (whether findings can be extended to other contexts) as they involve real-world interventions instead of controlled interventions in artificial laboratory settings.

Despite the distinct difference between true and quasi-experimental research designs, these two research methodologies share the following aspects:

Both study methods subject participants to some form of treatment or conditions.

Researchers have the freedom to measure some of the outcomes of interest.

Researchers can test whether the differences in the outcomes are associated with the treatment.

• An example comparing a true experiment and quasi-experiment

Imagine you wanted to study the effects of junk food on obese people. Here's how you would do this as a true experiment and a quasi-experiment:

How to carry out a true experiment

In a true experiment, some participants would eat junk foods, while the rest would be in the control group, adhering to a regular diet. At the end of the study, you would record the health and discomfort of each group.

This kind of experiment would raise ethical concerns since the participants assigned to the treatment group are required to eat junk food against their will throughout the experiment. This calls for a quasi-experimental design.

How to carry out a quasi-experiment

In quasi-experimental research, you would start by finding out which participants want to try junk food and which prefer to stick to a regular diet. This allows you to assign these two groups based on subject choice.

In this case, you didn't assign participants to a particular group, so you can confidently use the results from the study.

When is a quasi-experimental design used?

Quasi-experimental designs are used when researchers don’t want to use randomization when evaluating their intervention.

What are the characteristics of quasi-experimental designs?

Some of the characteristics of a quasi-experimental design are:

Researchers don't randomly assign participants into groups, but study their existing characteristics and assign them accordingly.

Researchers study the participants in pre- and post-testing to determine the progress of the groups.

Quasi-experimental design is ethical since it doesn’t involve offering or withholding treatment at random.

Quasi-experimental design encompasses a broad range of non-randomized intervention studies. This design is employed when it is not ethical or logistically feasible to conduct randomized controlled trials. Researchers typically employ it when evaluating policy or educational interventions, or in medical or therapy scenarios.

How do you analyze data in a quasi-experimental design?

You can use two-group tests, time-series analysis, and regression analysis to analyze data in a quasi-experiment design. Each option has specific assumptions, strengths, limitations, and data requirements.

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Quasi-Experimental Design: Rigor Meets Real-World Conditions

Experimental designs provide researchers with a powerful tool to infer cause-and-effect relationships, ensuring external variables are controlled, and thereby enhancing the reliability and validity of the results. But what happens when a purely experimental setup is neither feasible nor ethical? This is where the quasi-experimental design comes into play.

Just as architects use different blueprints for buildings based on their purpose and location, researchers employ various methodologies tailored to their study's needs. Many consider the Completely Randomized Design , a type of "true experimental design," as the gold standard. In this approach, the randomization of variables is paramount to ensure that underlying differences between groups don't obscure causality conclusions.

To simplify, researchers tweak certain factors (independent variables) intentionally to observe changes in another variable (the dependent one) . By randomizing these independent variables across participant groups, potential biases are minimized, and the study's validity is bolstered. But, what if randomizing isn't an option?

In situations where it's impractical or unethical to randomize, such as evaluating the impact of a new health policy on specific demographics, the quasi-experimental design shines. The pivotal difference? Quasi-experimental designs do not hinge on randomization. They're the go-to when randomization isn't feasible.

As we delve into the intricacies of experimental and quasi-experimental designs, it's important to understand the distinction between "random assignment" and "random sampling." While both terms involve randomization, they serve different purposes in research.

• Random Assignment: This refers to the random allocation of participants into different groups, such as treatment and comparison groups. It ensures that any pre-existing differences among participants are evenly distributed across groups, thus enhancing the validity of causal inferences.
• Random Sampling: This pertains to how participants are selected from a larger population for inclusion in a study. A random sample is drawn such that every individual in the population has an equal chance of being chosen, which bolsters the generalizability of the study results to the larger population.

While random sampling influences who is in a study, random assignment affects the group to which a participant is allocated once they are in the study. It's essential to distinguish between these two to appreciate the methodologies' nuances discussed.

Quasi-experimental designs, by nature, often lack the component of random assignment, which is a cornerstone in true experiments for making strong causal inferences. This absence can render the conclusions from quasi-experiments less definitive regarding cause and effect. However, it's important to note that while they might not involve random assignment to groups, quasi-experimental designs can still utilize random sampling when selecting participants from a larger population. This ensures that the sample represents the broader group, even if the allocation to specific conditions within the study isn't randomized.

Quasi-experiments across fields

The versatility of quasi-experimental design extends across numerous disciplines, each leveraging its flexibility and adaptability to explore a variety of complex issues. Here are some key areas where this design proves invaluable:

• Education: Gauging the effectiveness of new teaching techniques, curriculum shifts, or education-centric interventions.
• Healthcare: In healthcare, this design is used especially when it's unethical or impractical to randomize patients into treatment groups. For instance, certain National Institutes of Health clinical trials deploy this method .
• Economics: Analyzing the intricate dynamics of real-world economic scenarios.
• Psychology: Investigating subjects that defy random assignment, like the influence of specific traumas or inherent personality traits on behavior.
• Environmental Science: Ideal for scenarios where controlled experiments on ecosystems or organic processes aren't feasible.
• Public Policy: Assessing the efficacy of governmental policies and programs, from housing initiatives to justice system reforms.
• Business and Marketing: Delving into the intricate factors influencing consumer behaviors.
• Developmental Studies: Employed when the welfare of child subjects is paramount and they can't be subjected to detrimental conditions.
• Criminal Justice: Evaluating a multifaceted system deeply interwoven with socio-political constructs.

While the hard sciences might seldom turn to quasi-experimental designs, the landscape is quite different in social sciences. There, they are invaluable, providing a window into human behavior patterns unattainable with strict, randomized experimental designs. According to UNICEF's Research Office , quasi-experimental designs are ideal for studying the post-implementation effects of programs or policies. In essence, when assessing policy impacts, quasi-experimental design is your best bet.

Types of quasi-experimental designs

When choosing the most appropriate research approach, you'll come across three primary quasi-experimental designs:

• Nonequivalent Groups Design: This design involves comparing two groups that aren't formed through random assignment.
• Time-Series Design: In this approach, measurements are taken at various intervals before and after an intervention.
• Pretest-Posttest Design: As the name indicates, measurements are taken both before and after the intervention to determine its impact.

Illustrative scenarios

To better understand the practical applications of various quasi-experimental designs, let's delve into a few real-world scenarios spanning different fields.

• Education: Suppose you're evaluating a new educational program. While it might seem logical to randomly assign it to different student groups, this could inadvertently offer an advantage or disadvantage to some. A more balanced method is the nonequivalent groups design. Select two comparable schools within a district: implement the new program in one, while the other retains the conventional curriculum. A comparison of scores before and after this quasi-experiment can demonstrate the new program's effectiveness.
• Healthcare: Consider public health interventions, such as vaccination campaigns. Ethical dilemmas emerge when deciding who receives potentially life-saving medicine purely for research. In this context, a time-series design is suitable. Documenting disease incidence rates in the population before and after vaccination sheds light on the campaign's effectiveness. This design captures changes in the dependent variable over a prolonged period.
• Workplace: When evaluating a stress-reduction program at work, the pretest-posttest design is ideal. Assess the dependent variable (employee stress levels) before and after participation in the program. Unlike the time-series design, which observes changes over a longer duration, this approach focuses on immediate impacts or reactions.

Participant selection steps

In any quasi-experimental design, the careful selection of participants is crucial to ensure the study's validity and reliability . Given the importance of this aspect, selecting the right participants becomes pivotal for the success and validity of the quasi-experiment. Let's break down the steps involved in this process.

• Sample Size: Ensure your sample adequately represents the target population, while minimizing potential confounding variables.
• Comparison Group: Despite the absence of randomization in quasi-experiments, it's crucial to identify a suitable comparison group. Ideally, experimental and comparison groups should be as similar as feasible.
• Selecting Variables: Choose variables that closely relate to your study's objectives, can be reliably measured, and can be controlled as much as possible.

Reflecting on the educational example, utilizing the nonequivalent groups design necessitates that chosen schools bear resemblances in demographics, policies, and overall structure. Comparing a K-5 elementary school with a K-12 mixed school isn't as insightful as juxtaposing two schools catering to identical grades. While you can control this discrepancy by focusing solely on K-5 students in the mixed school, the overarching objective remains: to achieve as much group equivalency as practical . It's imperative to recognize that, unlike controlled lab experiments, achieving total control isn't always feasible.

Advantages of quasi-experimental design

In many situations, a quasi-experimental design can be as effective as, or even more so than, a true experimental design. Its ability to infer causality without the need for a randomly assigned comparison group makes it a versatile alternative. The primary strengths of quasi-experimental designs include:

• Applicability in Real-world Settings: Quasi-experimental designs are particularly suited for real-world environments. Unlike true experiments that may require artificial conditions, these designs yield results that more closely reflect real-life situations. For instance, consider a city planning to implement a new traffic management system to reduce congestion. Directly altering traffic patterns in various parts of the city simultaneously could disrupt daily commutes and cause confusion. However, with a quasi-experimental design, areas where the new traffic system has been implemented can be compared with areas still using the older system. This approach offers valuable insights into the effectiveness of the new system without causing widespread disruption to city residents.
• Cost and Time Efficiency: Conducting research in strictly controlled settings can be both time-consuming and costly. By sidestepping the strict requirements of true experimental designs, quasi-experimental methods offer researchers more flexibility, often leading to savings in time and money. For instance, a company looking to assess a new training program's effect on employee performance might find a traditional controlled experiment too expensive and disruptive. A quasi-experimental design could compare productivity levels before and after the training, saving both time and resources.
• Ethical Sensitivity: Traditional experimental approaches sometimes pose ethical challenges , especially when random assignment could harm participants. Quasi-experimental designs, by using existing groups or conditions, avoid these ethical concerns. To illustrate, a health researcher studying the benefits of exercise for heart surgery patients would face ethical issues if some patients were randomly prevented from exercising. A quasi-experimental approach could compare the recovery of patients who choose to exercise with those who don't, ensuring no one is forced into or denied any treatment.

By capitalizing on these strengths, quasi-experimental designs provide researchers with a balance of rigor and adaptability, proving invaluable across various research areas.

Limitations of quasi-experimental design

Despite the valuable insights offered by quasi-experimental designs, they come with certain limitations that researchers should be wary of. Chief among these are the potential for confounding variables and concerns related to internal validity.

• Potential for Confounding Variables: Confounding variables are external factors that can influence the relationship between the independent and dependent variables, thereby obscuring genuine causality. These are neither the variables being manipulated nor the outcomes being measured, but they can interfere with the interpretation of results. For example, consider a study investigating the link between coffee consumption and heart disease risk. If the study doesn't account for other lifestyle habits like smoking or exercise patterns, these factors can act as confounding variables. In such a scenario, it becomes challenging to determine whether heart disease is influenced by coffee intake or these other habits. Therefore, without controlling for confounding variables, drawing valid conclusions about causality is problematic.
• Concerns about Internal Validity: Internal validity reflects the degree to which the observed effects in a study are solely attributed to changes in the independent variable and not by external interferences. In essence, it ensures that the study accurately measures what it intends to without distortions from outside factors. Quasi-experimental designs sometimes struggle with ensuring high internal validity because they lack random assignment, which can make results less reliable or valid. For instance, a municipality decides to implement a new policy where they increase the frequency of garbage collection in an effort to reduce litter on the streets. After the policy change, they observe a noticeable decrease in street litter. However, during the same period, a major environmental awareness campaign was launched by a local NGO, urging residents to reduce, reuse, and recycle. In this context, it becomes challenging to determine if the decrease in street litter is primarily due to the increased garbage collection frequency or influenced significantly by the environmental campaign.

In understanding quasi-experimental designs, it's imperative to weigh these limitations against the method's inherent strengths, ensuring a comprehensive perspective on its applicability in research scenarios.

Case studies illustrating quasi-experimental designs

Let's look at a few real-world quasi-experimental case studies. These case studies highlight the nuanced applications of quasi-experimental designs in understanding real-world scenarios. While these designs may not always offer the rigorous causality of true experiments, their findings are often instrumental in shaping policies, interventions, and strategies across sectors.

Nonequivalent groups design

• The Oregon Health Insurance Experiment : In 2008, Oregon used a lottery system to distribute limited Medicaid slots to uninsured residents, leading to the Oregon Health Insurance Experiment (OHIE). This quasi-experimental design compared the outcomes of those who received Medicaid via the lottery with those who didn't, offering insights into the effects of Medicaid. Results showed Medicaid recipients used more healthcare services, experienced reduced financial strain, reported better self-perceived health, and saw a significant reduction in depression occurrence. However, certain physical health measures didn't show significant improvements over the study's two-year span, and the study's findings, though robust, were specific to Oregon's context.
• Moving to Opportunity Experiment : In the 1990s, the U.S. Department of Housing and Urban Development initiated the Moving to Opportunity (MTO) experiment to understand the effects of residential relocation on families from high-poverty urban settings. Families selected via a lottery system were given the opportunity to move to lower-poverty neighborhoods, establishing a quasi-experimental design where their progress in areas like employment, income, education, and health was compared to those who remained in high-poverty areas. The results from MTO indicated significant improvements in mental and physical well-being among the relocators, especially in women and younger children. Additionally, young adults who moved exhibited higher incomes and greater college attendance rates compared to their counterparts who didn't move. This landmark study underscored the profound long-term impact of neighborhood environments on socio-economic and health outcomes, bolstering the case for housing mobility programs as a policy tool for breaking cycles of urban poverty.
• Operation Peacemaker Fellowship : In Richmond, California, policymakers took a unique stance to curb gun violence with the introduction of a program that provided financial stipends to individuals deemed likely to engage in gun-related offenses. This wasn't just a straightforward financial transaction; in exchange for the stipend, recipients were required to participate in mentorship and personal development initiatives aimed at promoting behavioral change and community integration. The effectiveness of this innovative strategy was evaluated by researchers who tracked the outcomes of the program's participants, focusing on metrics such as their involvement in subsequent shootings or any re-arrests. For a more comprehensive analysis, they contrasted these results with those from a comparable group of at-risk individuals who did not enroll in the program. This juxtaposition offered insights into whether the combined approach of financial incentives and structured mentorship could effectively deter potential offenders from engaging in gun violence.

Time-series design

• London Congestion Charging Impact : In 2003, London introduced a congestion charge, requiring motorists to pay a fee when driving in central London during certain hours. Using a Time-Series Design, researchers observed traffic volumes, air quality, and public transportation usage before and after the implementation of the charge. The data showed not only a substantial reduction in traffic volumes within the charging zone but also improvements in air quality and increased public transportation use. This served as empirical evidence for the benefits of congestion pricing both in reducing traffic and potentially in improving urban air quality.
• Impact of Public Smoking Bans : As concerns over the health implications of passive smoking grew globally, numerous countries and cities proactively instituted bans on public smoking. In an effort to discern the tangible impacts of these bans, researchers turned to Time-Series Designs to examine hospital admission trends related to smoking-associated illnesses both before and after the introduction of the prohibitions. A consistent pattern that emerged from multiple studies was a marked reduction in hospitalizations for conditions like heart attacks, chronic obstructive pulmonary diseases, and asthma post-implementation of the bans. Beyond just establishing a correlation, these findings presented compelling evidence of the immediate and tangible health benefits derived from such policies, effectively underlining the crucial role of legislative interventions in enhancing public health and reducing healthcare burdens.
• Los Angeles Air Quality Analysis : In response to rising concerns over deteriorating air quality and its subsequent health implications, Los Angeles instituted a series of stringent emission-reducing policies spanning several decades. The city, once notorious for its smog and pollution, became a focal point for scientists aiming to quantify the results of these environmental strategies. Leveraging Time-Series Designs, researchers have charted the levels of various pollutants over extended periods, juxtaposing periods before and after the implementation of specific policies. For instance, a detailed study by the South Coast Air Quality Management District showcased that from the 1980s to recent years, there has been a notable decrease in the concentration of ground-level ozone, particulate matter, and other harmful pollutants.

Pretest-posttest design

• Head Start Program Evaluation : The Head Start program, initiated in the 1960s, is a U.S. federal program that aims to promote school readiness of children under 5 from low-income families through education, health, social, and other services. To assess the effectiveness of the program, researchers often use a Pretest-Posttest Design. Before entering the program (pretest), children are assessed on various cognitive, social, and health measures. After participating in the program, they are assessed again (posttest). Over the years, evaluations of the program have shown mixed results. Some studies find significant short-term cognitive and social gains for children in the program, but many of these gains diminish by the time the children reach elementary school.
• D.A.R.E. Program Evaluation : D.A.R.E. is a school-based drug use prevention program that was widely implemented in schools across the U.S. starting in the 1980s. The program's curriculum aims to teach students good decision-making skills to help them lead safe and healthy lives. To assess its effectiveness, numerous evaluations have been conducted using a Pretest-Posttest Design. Before participating in the D.A.R.E. program (pretest), students are surveyed regarding their attitudes toward drugs and their self-reported drug use. After completing the program, students are surveyed again (posttest). Over the years, the evaluations have yielded mixed results. While some studies suggest the program improves students' knowledge and attitudes about drugs, other research indicates limited or no long-term impact on actual drug use.
• Cognitive-Behavioral Therapy for Anxiety Disorders : Cognitive-behavioral therapy (CBT) is a common treatment approach for individuals with anxiety disorders. To evaluate its effectiveness, many studies employ a Pretest-Posttest Design. Before undergoing CBT (pretest), individuals' levels of anxiety are assessed using standardized measures, such as the Beck Anxiety Inventory (BAI) . After completing a series of CBT sessions, these individuals are reassessed (posttest) to measure any changes in their anxiety levels. Numerous studies have consistently shown that CBT can lead to significant reductions in symptoms of anxiety, highlighting its efficacy as a treatment modality.

Analyzing data from quasi-experiments

Quasi-experimental designs, by nature, present inherent constraints that make data analysis particularly challenging. In response, researchers utilize a range of techniques designed to enhance the accuracy and relevance of their findings. These techniques encompass specific statistical methods to control for bias, supplementary research to corroborate initial results, and cross-referencing with external data to validate causality.

Statistical methods

Several key statistical methods are particularly relevant for quasi-experimental research. These methods play a pivotal role in refining and enhancing the quality of the findings.

• Regression Analysis : This technique identifies relationships between variables. It involves plotting data points from these variables and drawing a line of best fit. By examining the patterns revealed by this line, researchers can discern trends and make predictions.
• Matching : Here, control groups are paired with experimental groups for comparison. Participants are grouped based on specific criteria, such as age or profession, to account for potential confounding variables. This enhances the internal validity of the study. However, this method can sometimes introduce selection bias.
• Interrupted Time Series Analysis : This method examines statistical differences observed before and after an intervention. Particularly useful when evaluating multiple data sets before and after an intervention, it helps determine the intervention's effectiveness and potential lasting effects. This is achieved by plotting data points over time, covering the period before, during, and after the intervention, which aids in assessing the intervention's impact on observed patterns.

By leveraging these methods in the appropriate contexts, researchers can achieve a deeper understanding and more robust conclusions from their quasi-experimental data.

Interpretation of results

Interpreting the results of quasi-experimental research is as critical as the data collection process itself. A proper understanding and interpretation can bridge the gap between raw data and actionable insights.

• Study Design and Data Collection Review: Researchers should begin with a thorough examination of the research design . Consider how effectively it controls for potential confounding variables. Studies that employ techniques such as randomization or matching to equate groups often yield more reliable results. It's equally important to assess the methods used for data collection. The use of standardized and validated instruments, along with appropriate data collection protocols, lends credibility to the results.
• Internal Validity: This pertains to the degree to which the results of the study accurately represent the true relationship between the variables in the absence of confounding factors. High internal validity indicates that the observed effects can confidently be attributed to the intervention or treatment, rather than external influences.
• External Validity: This concerns the generalizability of the study's results. While a study might have strong internal validity, its findings might not necessarily apply to wider or different populations or settings. Researchers should reflect on the boundaries of their study and the contexts in which their findings can be generalized.
• Balance Between Internal and External Validity: Navigating the balance between internal and external validity is pivotal. While ensuring rigorous controls boosts internal validity, it might restrict the findings' broader applicability. Conversely, focusing on external validity might compromise the accuracy of the causal relationships being studied. Researchers must be aware of this delicate balance, ensuring results are both reliable and applicable. This involves a conscious evaluation of trade-offs and tailoring the research design to meet study objectives.
• Statistical Significance vs. Practical Significance: While a result may be statistically significant, its practical, real-world impact might not always be meaningful. Researchers should differentiate between these two to avoid over- or underestimating the implications of their findings.
• Multicollinearity: In research models involving multiple independent variables, multicollinearity arises when two or more variables are closely correlated with each other. This can make it challenging to determine the individual effect of each variable on the outcome. For instance, in a study examining the factors affecting a student's academic performance, if many students who spend more hours studying also attend additional tutoring sessions, it becomes difficult to isolate which factor—study hours or tutoring—is having a more pronounced impact on their grades.
• Avoiding the Ecological Fallacy: When interpreting group-level data, researchers must be careful not to infer that relationships observed for groups necessarily hold for individuals within those groups. The ecological fallacy arises when conclusions about individuals are drawn based on group-level data. For instance, if a study finds a relationship between average income levels in a region and average educational attainment, it would be fallacious to conclude that every individual with higher income in that region has a higher educational attainment. Researchers must be cautious and ensure they do not overextend their conclusions beyond the data's scope.
• Bias and Limitations Acknowledgment: No study is without its limitations. Recognizing and addressing potential biases, shortcomings, or areas of improvement in the research design and execution is essential for a comprehensive interpretation. Transparent communication of these elements not only enhances the credibility of the study but also provides a roadmap for future research.

The interpretation phase is where data is transformed into knowledge. Researchers must approach this stage with a blend of rigor, skepticism, and openness to ensure their findings are both trustworthy and valuable to the broader scientific community and real-world applications.

Understanding bias in quasi-experimental design

Bias in quasi-experimental studies refers to the distortion of results. It can manifest in various ways, potentially skewing the conclusions drawn from the research. It's crucial to recognize and mitigate these biases to ensure that the findings of a study are reliable and valid.

• Definition: Measurement bias arises from systematic errors in the measurement process, leading to skewed or inaccurate results. This can happen if the instruments or methods used for measuring deviate consistently from the true value of what's being measured.
• Example: Suppose a researcher is evaluating a new teaching technique by comparing student test scores before and after its application. If the post-test is inherently easier than the pre-test, the post-test scores may be artificially high. This scenario would inaccurately suggest that the new teaching method is highly effective.
• Mitigation: To counteract measurement bias, researchers should employ standardized tools and ensure that the same equipment and procedures are used consistently across all participants.
• Definition: Selection bias is introduced when the sample selected for a study doesn't accurately represent the broader population. This can result in findings that are not generalizable.
• Example: Consider a study assessing the efficacy of a new medication. If participants who receive the medication are self-selected and inherently more motivated to recover, the results might overstate the drug's effectiveness.
• Mitigation: To reduce selection bias, it's essential to carefully choose participants who accurately reflect the population under investigation.
• Definition: Recall bias occurs when participants' memories of past events or experiences aren't consistent or accurate, leading to skewed data based on these recollections.
• Example: In a study examining the impact of a specific diet on weight loss, if participants are asked to recall their food consumption over the past week, those following the diet might be more conscious and thus recall their intake more accurately than those not on the diet. This could exaggerate the perceived effectiveness of the diet.
• Mitigation: To minimize recall bias, researchers should rely more on objective behavioral or outcome measures rather than solely on self-reported data. Regular check-ins with participants can also help ensure that their recall remains consistent and reliable.
• Definition: Confounding bias occurs when an external factor, not considered in the study, affects both the independent and dependent variables. This can lead to mistaken conclusions about the cause-and-effect relationship.
• Example: In a study examining the impact of exercise on mood improvement, if participants who exercised also spent more time outdoors, and exposure to natural light is a mood enhancer, then the mood improvement might be wrongly attributed entirely to exercise without considering the impact of natural light.
• Mitigation: To address confounding bias, researchers can use techniques like stratification or multivariate analysis to account for potential confounding variables.

By recognizing and addressing these biases, researchers can increase the validity of their quasi-experimental studies, ensuring that the conclusions drawn are both accurate and meaningful.

Quasi-experimental research offers a valuable approach for investigating complex real-world phenomena in their natural settings. This method's flexibility allows for variable manipulation within authentic contexts, proving especially beneficial when ethical or logistical constraints rule out true experimental studies. This approach is crucial for establishing causal relationships and garnering insights from practical situations. It also holds significant value across various fields, including education, healthcare, business, and marketing. The adaptability of quasi-experimental research makes it a favored alternative when traditional experimental designs are impractical.

However, as with all research methods, quasi-experimental designs have their limitations. A primary concern is their susceptibility to confounding variables that can inadvertently influence results. Furthermore, drawing causal inferences becomes more challenging due to the reduced rigor and control, compared to traditional experimental designs. Thus, when considering this approach, researchers must remain cognizant of these limitations.

For those diving into quasi-experimental research, it's essential to thoughtfully match experimental groups and utilize rigorous statistical analyses. This attention to detail aids in minimizing biases and potential errors, ensuring more dependable data and conclusions.

In summary, quasi-experimental methods provide researchers with robust tools for gauging intervention efficacy and deciphering the intricate dynamics of variables. These methods remain a vital component of the research arsenal, guiding informed decision-making.

Header image by Krakenimages .

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