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• Quasi-Experimental Design | Definition, Types & Examples

Quasi-Experimental Design | Definition, Types & Examples

Published on July 31, 2020 by Lauren Thomas . Revised on January 22, 2024.

Like a true experiment , a quasi-experimental design aims to establish a cause-and-effect relationship between an independent and dependent variable .

However, unlike a true experiment, a quasi-experiment does not rely on random assignment . Instead, subjects are assigned to groups based on non-random criteria.

Quasi-experimental design is a useful tool in situations where true experiments cannot be used for ethical or practical reasons.

Table of contents

Differences between quasi-experiments and true experiments, types of quasi-experimental designs, when to use quasi-experimental design, advantages and disadvantages, other interesting articles, frequently asked questions about quasi-experimental designs.

There are several common differences between true and quasi-experimental designs.

Example of a true experiment vs a quasi-experiment

However, for ethical reasons, the directors of the mental health clinic may not give you permission to randomly assign their patients to treatments. In this case, you cannot run a true experiment.

Instead, you can use a quasi-experimental design.

You can use these pre-existing groups to study the symptom progression of the patients treated with the new therapy versus those receiving the standard course of treatment.

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Many types of quasi-experimental designs exist. Here we explain three of the most common types: nonequivalent groups design, regression discontinuity, and natural experiments.

Nonequivalent groups design

In nonequivalent group design, the researcher chooses existing groups that appear similar, but where only one of the groups experiences the treatment.

In a true experiment with random assignment , the control and treatment groups are considered equivalent in every way other than the treatment. But in a quasi-experiment where the groups are not random, they may differ in other ways—they are nonequivalent groups .

When using this kind of design, researchers try to account for any confounding variables by controlling for them in their analysis or by choosing groups that are as similar as possible.

This is the most common type of quasi-experimental design.

Regression discontinuity

Many potential treatments that researchers wish to study are designed around an essentially arbitrary cutoff, where those above the threshold receive the treatment and those below it do not.

Near this threshold, the differences between the two groups are often so minimal as to be nearly nonexistent. Therefore, researchers can use individuals just below the threshold as a control group and those just above as a treatment group.

However, since the exact cutoff score is arbitrary, the students near the threshold—those who just barely pass the exam and those who fail by a very small margin—tend to be very similar, with the small differences in their scores mostly due to random chance. You can therefore conclude that any outcome differences must come from the school they attended.

Natural experiments

In both laboratory and field experiments, researchers normally control which group the subjects are assigned to. In a natural experiment, an external event or situation (“nature”) results in the random or random-like assignment of subjects to the treatment group.

Even though some use random assignments, natural experiments are not considered to be true experiments because they are observational in nature.

Although the researchers have no control over the independent variable , they can exploit this event after the fact to study the effect of the treatment.

However, as they could not afford to cover everyone who they deemed eligible for the program, they instead allocated spots in the program based on a random lottery.

Although true experiments have higher internal validity , you might choose to use a quasi-experimental design for ethical or practical reasons.

Sometimes it would be unethical to provide or withhold a treatment on a random basis, so a true experiment is not feasible. In this case, a quasi-experiment can allow you to study the same causal relationship without the ethical issues.

The Oregon Health Study is a good example. It would be unethical to randomly provide some people with health insurance but purposely prevent others from receiving it solely for the purposes of research.

However, since the Oregon government faced financial constraints and decided to provide health insurance via lottery, studying this event after the fact is a much more ethical approach to studying the same problem.

True experimental design may be infeasible to implement or simply too expensive, particularly for researchers without access to large funding streams.

At other times, too much work is involved in recruiting and properly designing an experimental intervention for an adequate number of subjects to justify a true experiment.

In either case, quasi-experimental designs allow you to study the question by taking advantage of data that has previously been paid for or collected by others (often the government).

Quasi-experimental designs have various pros and cons compared to other types of studies.

• Higher external validity than most true experiments, because they often involve real-world interventions instead of artificial laboratory settings.
• Higher internal validity than other non-experimental types of research, because they allow you to better control for confounding variables than other types of studies do.
• Lower internal validity than true experiments—without randomization, it can be difficult to verify that all confounding variables have been accounted for.
• The use of retrospective data that has already been collected for other purposes can be inaccurate, incomplete or difficult to access.

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.

• Normal distribution
• Degrees of freedom
• Null hypothesis
• Discourse analysis
• Control groups
• Mixed methods research
• Non-probability sampling
• Quantitative research
• Ecological validity

Research bias

• Rosenthal effect
• Implicit bias
• Cognitive bias
• Selection bias
• Negativity bias
• Status quo bias

A quasi-experiment is a type of research design that attempts to establish a cause-and-effect relationship. The main difference with a true experiment is that the groups are not randomly assigned.

In experimental research, random assignment is a way of placing participants from your sample into different groups using randomization. With this method, every member of the sample has a known or equal chance of being placed in a control group or an experimental group.

Quasi-experimental design is most useful in situations where it would be unethical or impractical to run a true experiment .

Quasi-experiments have lower internal validity than true experiments, but they often have higher external validity  as they can use real-world interventions instead of artificial laboratory settings.

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Statistics By Jim

Making statistics intuitive

Quasi Experimental Design Overview & Examples

By Jim Frost Leave a Comment

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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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.

How QuestionPro helps in quasi-experimental research?

QuestionPro can be a useful tool in quasi-experimental research because it includes features that can assist you in designing and analyzing your research study. Here are some ways in which QuestionPro can help in quasi-experimental research:

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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.

5 Quasi-Experimental Design Examples

Dave Cornell (PhD)

Dr. Cornell has worked in education for more than 20 years. His work has involved designing teacher certification for Trinity College in London and in-service training for state governments in the United States. He has trained kindergarten teachers in 8 countries and helped businessmen and women open baby centers and kindergartens in 3 countries.

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Chris Drew (PhD)

This article was peer-reviewed and edited by Chris Drew (PhD). The review process on Helpful Professor involves having a PhD level expert fact check, edit, and contribute to articles. Reviewers ensure all content reflects expert academic consensus and is backed up with reference to academic studies. Dr. Drew has published over 20 academic articles in scholarly journals. He is the former editor of the Journal of Learning Development in Higher Education and holds a PhD in Education from ACU.

Quasi-experimental design refers to a type of experimental design that uses pre-existing groups of people rather than random groups.

Because the groups of research participants already exist, they cannot be randomly assigned to a cohort . This makes inferring a causal relationship between the treatment and observed/criterion variable difficult.

Quasi-experimental designs are generally considered inferior to true experimental designs.

Limitations of Quasi-Experimental Design

Since participants cannot be randomly assigned to the grouping variable (male/female; high education/low education), the internal validity of the study is questionable.

Extraneous variables may exist that explain the results. For example, with quasi-experimental studies involving gender, there are numerous cultural and biological variables that distinguish males and females other than gender alone.

Each one of those variables may be able to explain the results without the need to refer to gender.

See More Research Limitations Here

Quasi-Experimental Design Examples

1. smartboard apps and math.

A school has decided to supplement their math resources with smartboard applications. The math teachers research the apps available and then choose two apps for each grade level. Before deciding on which apps to purchase, the school contacts the seller and asks for permission to demo/test the apps before purchasing the licenses.

The study involves having different teachers use the apps with their classes. Since there are two math teachers at each grade level, each teacher will use one of the apps in their classroom for three months. At the end of three months, all students will take the same math exams. Then the school can simply compare which app improved the students’ math scores the most.

The reason this is called a quasi-experiment is because the school did not randomly assign students to one app or the other. The students were already in pre-existing groups/classes.

Although it was impractical to randomly assign students to use one version or the other of the apps, it creates difficulty interpreting the results.

For instance, if students in teacher A’s class did better than the students in teacher B’s class, then can we really say the difference was due to the app? There may be other differences between the two teachers that account for the results. This poses a serious threat to the study’s internal validity.

2. Leadership Training

There is reason to believe that teaching entrepreneurs modern leadership techniques will improve their performance and shorten how long it takes for them to reach profitability. Team members will feel better appreciated and work harder, which should translate to increased productivity and innovation.

This hypothetical study took place in a third-world country in a mid-sized city. The researchers marketed the training throughout the city and received interest from 5 start-ups in the tech sector and 5 in the textile industry. The leaders of each company then attended six weeks of workshops on employee motivation, leadership styles, and effective team management.

At the end of one year, the researchers returned. They conducted a standard assessment of each start-up’s growth trajectory and administered various surveys to employees.

The results indicated that tech start-ups were further along in their growth paths than textile start-ups. The data also showed that tech work teams reported greater job satisfaction and company loyalty than textile work teams.

Although the results appear straightforward, because the researchers used a quasi-experimental design, they cannot say that the training caused the results.

The two groups may differ in ways that could explain the results. For instance, perhaps there is less growth potential in the textile industry in that city, or perhaps tech leaders are more progressive and willing to accept new leadership strategies.

3. Parenting Styles and Academic Performance   

Psychologists are very interested in factors that affect children’s academic performance. Since parenting styles affect a variety of children’s social and emotional profiles, it stands to reason that it may affect academic performance as well. The four parenting styles under study are: authoritarian, authoritative, permissive, and neglectful/uninvolved.

To examine this possible relationship, researchers assessed the parenting style of 120 families with third graders in a large metropolitan city. Trained raters made two-hour home visits to conduct observations of parent/child interactions. That data was later compared with the children’s grades.

The results revealed that children raised in authoritative households had the highest grades of all the groups.

However, because the researchers were not able to randomly assign children to one of the four parenting styles, the internal validity is called into question.

There may be other explanations for the results other than parenting style. For instance, maybe parents that practice authoritative parenting also come from a higher SES demographic than the other parents.

Because they have higher income and education levels, they may put more emphasis on their child’s academic performance. Or, because they have greater financial resources, their children attend STEM camps, co-curricular and other extracurricular academic-orientated classes.

4. Government Reforms and Economic Impact

Government policies can have a tremendous impact on economic development. Making it easier for small businesses to open and reducing bank loans are examples of policies that can have immediate results. So, a third-world country decides to test policy reforms in two mid-sized cities. One city receives reforms directed at small businesses, while the other receives reforms directed at luring foreign investment.  

The government was careful to choose two cities that were similar in terms of size and population demographics.

Over the next five years, economic growth data were collected at the end of each fiscal year. The measures consisted of housing sells, local GDP, and unemployment rates.

At the end of five years the results indicated that small business reforms had a much larger impact on economic growth than foreign investment. The city which received small business reforms saw an increase in housing sells and GDP, but a drop in unemployment. The other city saw stagnant sells and GDP, and a slight increase in unemployment.

On the surface, it appears that small business reform is the better way to go. However, a more careful analysis revealed that the economic improvement observed in the one city was actually the result of two multinational real estate firms entering the market. The two firms specialize in converting dilapidated warehouses into shopping centers and residential properties.

5. Gender and Meditation

Meditation can help relieve stress and reduce symptoms of depression and anxiety. It is a simple and easy to use technique that just about anyone can try. However, are the benefits real or is it just that people believe it can help? To find out, a team of counselors designed a study to put it to a test.

Since they believe that women are more likely to benefit than men, they recruit both males and females to be in their study.

Both groups were trained in meditation by a licensed professional. The training took place over three weekends. Participants were instructed to practice at home at least four times a week for the next three months and keep a journal each time they meditate.

At the end of the three months, physical and psychological health data were collected on all participants. For physical health, participants’ blood pressure was measured. For psychological health, participants filled out a happiness scale and the emotional tone of their diaries were examined.

The results showed that meditation worked better for women than men. Women had lower blood pressure, scored higher on the happiness scale, and wrote more positive statements in their diaries.

Unfortunately, the researchers noticed that men apparently did not actually practice meditation as much as they should. They had very few journal entries and in post-study interviews, a vast majority of men admitted that they only practiced meditation about half the time.

The lack of practice is an extraneous variable. Perhaps if men had adhered to the study instructions, their scores on the physical and psychological measures would have been higher than women’s measures.

The quasi-experiment is used when researchers want to study the effects of a variable/treatment on different groups of people. Groups can be defined based on gender, parenting style, SES demographics, or any number of other variables.

The problem is that when interpreting the results, even clear differences between the groups cannot be attributed to the treatment.

The groups may differ in ways other than the grouping variables. For example, leadership training in the study above may have improved the textile start-ups’ performance if the techniques had been applied at all. Similarly, men may have benefited from meditation as much as women, if they had just tried.

Baumrind, D. (1991). Parenting styles and adolescent development. In R. M. Lerner, A. C. Peterson, & J. Brooks-Gunn (Eds.), Encyclopedia of Adolescence (pp. 746–758). New York: Garland Publishing, Inc.

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

Matthew L. Maciejewski (2020) Quasi-experimental design. Biostatistics & Epidemiology, 4 (1), 38-47. https://doi.org/10.1080/24709360.2018.1477468

Thyer, Bruce. (2012). Quasi-Experimental Research Designs . Oxford University Press. https://doi.org/10.1093/acprof:oso/9780195387384.001.0001

• Dave Cornell (PhD) https://helpfulprofessor.com/author/dave-cornell-phd/ 23 Achieved Status Examples
• Dave Cornell (PhD) https://helpfulprofessor.com/author/dave-cornell-phd/ 25 Defense Mechanisms Examples
• Dave Cornell (PhD) https://helpfulprofessor.com/author/dave-cornell-phd/ 15 Theory of Planned Behavior Examples
• Dave Cornell (PhD) https://helpfulprofessor.com/author/dave-cornell-phd/ 18 Adaptive Behavior Examples

• Chris Drew (PhD) https://helpfulprofessor.com/author/chris-drew-phd/ 23 Achieved Status Examples
• Chris Drew (PhD) https://helpfulprofessor.com/author/chris-drew-phd/ 15 Ableism Examples
• Chris Drew (PhD) https://helpfulprofessor.com/author/chris-drew-phd/ 25 Defense Mechanisms Examples
• Chris Drew (PhD) https://helpfulprofessor.com/author/chris-drew-phd/ 15 Theory of Planned Behavior Examples

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Quasi-experimental study designs series—paper 5: a checklist for classifying studies evaluating the effects on health interventions—a taxonomy without labels

Barnaby c. reeves.

a Clinical Trials and Evaluation Unit, School of Clinical Sciences, University of Bristol, Level 7 Queen's Building, Bristol Royal Infirmary, Bristol BS2 8HW, UK

George A. Wells

b Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa Heart Institute, 40 Ruskin Street, Ottawa, Ontario, Canada K1Y 4W7

Hugh Waddington

c International Initiative for Impact Evaluation (3ie), 202-203, Rectangle One, D-4, Saket District Centre, New Delhi, 110017, India

The aim of the study was to extend a previously published checklist of study design features to include study designs often used by health systems researchers and economists. Our intention is to help review authors in any field to set eligibility criteria for studies to include in a systematic review that relate directly to the intrinsic strength of the studies in inferring causality. We also seek to clarify key equivalences and differences in terminology used by different research communities.

Study Design and Setting

Expert consensus meeting.

The checklist comprises seven questions, each with a list of response items, addressing: clustering of an intervention as an aspect of allocation or due to the intrinsic nature of the delivery of the intervention; for whom, and when, outcome data are available; how the intervention effect was estimated; the principle underlying control for confounding; how groups were formed; the features of a study carried out after it was designed; and the variables measured before intervention.

The checklist clarifies the basis of credible quasi-experimental studies, reconciling different terminology used in different fields of investigation and facilitating communications across research communities. By applying the checklist, review authors' attention is also directed to the assumptions underpinning the methods for inferring causality.

What is new?

• • Evaluations of health system interventions have features that differ and which are described differently compared to evaluations of health care interventions.
• • An existing checklist of features has been extended to characterize: nesting of data in organizational clusters, for example, service providers; number of outcome measurements and whether outcomes were measured in the same or different individuals; whether the effects of an intervention are estimated by change over time or between groups; and the intrinsic ability of the analysis to control for confounding.
• • Evaluations of health care and health system interventions have features that affect their credibility with respect to establishing causality but which are not captured by study design labels.
• • With respect to inferring causality, review authors need to consider these features to discriminate “strong” from “weak” designs.
• • Review authors can define eligibility criteria for a systematic review with reference to these study design features, but applying the checklist does not obviate the need for a careful risk of bias assessment.

1. Introduction

There are difficulties in drawing up a taxonomy of study designs to evaluate health care interventions or systems that do not use randomization [1] . To avoid the ambiguities of study design labels, a checklist of design features has been proposed by the Cochrane Non-Randomized Studies Methods Group (including B.C.R. and G.A.W.) to classify nonrandomized studies of health care interventions on the basis of what researchers did [1] , [2] . The checklist includes items about: whether a study made a comparison and, if yes, how comparison groups were formed; the timing of key elements of a study in relation to its conduct; and variables compared between intervention and comparator groups [1] , [2] . The checklist was created primarily from the perspective of health care evaluation, that is, the kinds of intervention most commonly considered in Cochrane reviews of interventions.

The checklist works well in principle for study designs in which the allocation mechanism applies to individual participants, although it does not characterize unit of analysis issues that may arise from the mechanism of allocation or the organizational hierarchy through which an intervention is provided (clustering by practitioner or organizational unit on which allocation is based), unit of treatment issues arising from the organizational hierarchy through which the intervention is provided, or unit of analysis issues arising from the unit at which data are collected and analysed (whether patient, practitioner or organisational aggregate). Most health interventions are delivered by discrete care provider units, typically organized hierarchically (e.g., hospitals, family practices, practitioners); this makes clustering important, except when allocation is randomized, because interventions are chosen by care provider units in complex ways. A modified checklist was also suggested for cluster-allocated designs (diverse study designs in which the allocation mechanism applies to groups of participants) [1] , [2] , often used to evaluate interventions applied at the level of the group (e.g., disease prevention, health education, health policy), but the authors acknowledged that this checklist had not been well piloted.

There are three key challenges when trying to communicate study designs that do not use randomization to evaluate the effectiveness of interventions. First, study design labels are diverse or ambiguous, especially for cluster-allocated designs; moreover, there are key differences between research fields in the way that similar designs are conceived. Second, some study designs are, in fact, strategies for analysis rather than designs per se. Terms such as quasi-experimental, natural experiment, and observational cause particular ambiguity. The current checklist does not explicitly consider designs/analyses commonly used in health systems research (including so-called “credible quasi-experimental studies” [3] , [4] ), often taking advantage of large administrative or other available data sets, and in other cases using data purposely collected as part of prospective designs where random assignment is not feasible. Third, and important with respect to the motivation for this paper, differences of opinion exist between health care and health systems researchers about the extent to which some studies are “as good as” randomized trials when well conducted; it is not clear whether this is because common designs are described with different labels or whether there are substantive differences. Therefore, our primary aim in this paper is revise the checklist to overcome these limitations.

Specific objectives were (1) to include a question to capture information about clustering; and (2) to extend the checklist to include study designs often used by health systems researchers and econometricians in a way that deals with the design/analysis challenge. We intended that the revised checklist should be able to resolve the differences in opinion about the extent to causality can be inferred from nonrandomized studies with different design features, improving communication between different health research communities. We did not intend that the checklist should be used as a tool to assess risk of bias, which can vary across studies with the same design features.

The paper is structured in three parts. Part 1 sets out designs currently used for health systems evaluations, illustrating their use through inclusion of different designs/analyses in a recent systematic review. Part 2 describes designs used for health intervention/program evaluations. Part 3 clarifies some of the ambiguities of study design labels using the proposed design feature framework.

2. Part 1: “quasi-experimental” studies considered by health system researchers and health economists

Health systems researchers and health economists use a wide range of “quasi-experimental” approaches to estimate causal effects of health care interventions. Some methods are considered stronger than others in estimating an unbiased causal relationship. “Credible quasi-experimental studies” are ones that “estimate a causal relationship using exogenous variation in the exposure of interest which is not usually directly controlled the researcher.” This exogenous variation refers to variation determined outside the system of relationships that are of interest and in some situations may be considered “as good as random” variation [3] , [4] , [5] . Credible quasi-experimental approaches are based on assignment to treatment and control that is not controlled by the investigators, and the term can be applied to different assignment rules; allocation to treatment and control is by definition not randomized, although some are based on identifying a source of variation in an exposure of interest that is assumed to be random (or exogenous). In the present context, they are considered to use rigorous designs and methods of analysis which can enable studies to adjust for unobservable sources of confounding [6] and are identical to the union of “strong” and “weak” quasi-experiments as defined by Rockers et al. [4] .

Credible quasi-experimental methods use assignment rules which are either known or can be modeled statistically, including: methods based on a threshold on a continuous scale (or ordinal scale with a minimum number of units) such as a test score (regression discontinuity design) or another form of “exogenous variation” arising, for example, due to geographical or administrative boundaries or assignment rules that have gone wrong (natural experiments). Quasi-experimental methods are also applied when assignment is self-selected by program administrators or by beneficiaries themselves [7] , [8] . Credible methods commonly used to identify causation among self-selected groups include instrumental variable estimation (IVE), difference studies [including difference in differences, (DIDs)] and, to a lesser extent, propensity score matching (PSM) where individuals or groups are matched on preexisting characteristics measured at baseline and interrupted time series (ITS). Thumbnail sketches of these and other designs used by health system researchers are described in Box 1 . It should be noted that the sketches of study types used by health program evaluators are not exhaustive. For example, pipeline studies, where treatment is withheld temporarily in one group until outcomes are measured (where time of treatment is not randomly allocated), are also used.

Thumbnail sketches of quasi-experimental studies used in program evaluations of CCT programs

Quasi-experimental methods are used increasingly to evaluate programs in health systems research. Gaarder et al. [11] , Baird et al. [12] , and Kabeer and Waddington [13] have published reviews incorporating quasi-experimental studies on conditional cash transfer (CCT) programs, which make welfare benefits conditional upon beneficiaries taking specified actions like attending a health facility during the pre/post-natal period or enrolling children in school. Other reviews including quasi-experimental studies have evaluated health insurance schemes [14] , [15] and maternal and child health programs [16] . Other papers in this themed issue of the Journal of Clinical Epidemiology describe how quasi-experimental studies can be identified for evidence synthesis [17] , how data are best collected from quasi-experimental studies [18] , and how the global capacity for including quasi-experimental studies in evidence synthesis can best be expanded [19] , [20] . In this paper, we use studies from the reviews on the effects of CCT programs to illustrate the wide range of quasi-experimental methods used to quantify causal effects of the programs ( Table 1 ).

Table 1

Experimental and quasi-experimental approaches applied in studies evaluating the effects of conditional cash transfer (CCT) programs

Some of the earliest CCT programs randomly assigned clusters (communities of households) and used longitudinal household survey data collected by researchers to estimate the effects of CCTs on the health of both adults and children [21] . The design and analysis of a cluster-randomized controlled trial of this kind is familiar to health care researchers [29] .

In other cases, it was not possible to assign beneficiaries randomly. In Jamaica's PATH program [22] , benefits were allocated to people with scores below a criterion level on a multidimensional deprivation index and the effects of the program were estimated using a regression discontinuity analysis. This study involved recruiting a cohort of participants being considered for benefits, to whom a policy decision was applied (i.e., assign benefits or not on the basis the specified deprivation threshold). In such studies, by assigning the intervention on the basis of a cutoff value for a covariate, the assignment mechanism (usually correlated with the outcome of interest) is completely known and can provide a strong basis for inferences, although usually in a less efficient manner than in randomized controlled trials (RCTs). The treatment effect is estimated as the difference (“discontinuity”) between two predictions of the outcome based on the covariate (the average treatment effect at the cutoff): one for individuals just above the covariate cutoff (control group) and one for individuals just below the cutoff (intervention group) [30] . The covariate is often a test score (e.g., to decide who receives a health or education intervention) [31] but can also be distance from a geographic boundary [32] . Challenges of this design are assignment determined approximately, but not perfectly, by the cutoff [33] or circumstances in which participants may be able to control factors determining their assignment status such as their score or location.

As with health care evaluation, many studies in health systems research combine multiple methods. In Ecuador's Bono de Desarrollo Humano program, leakages in implementation caused ineligible families to receive the program, compromising the original discontinuity assignment. To compensate for this problem, the effects of the program were estimated as a “fuzzy discontinuity” using IVE [23] . An instrument (in this case, a dichotomous variable taking the value of 1 or 0 depending on whether the participating family had a value on a proxy means test below or above a cutoff value used to determine eligibility to the program) must be associated with the assignment of interest, unrelated to potential confounding factors and related to the outcome of interest only by virtue of the relationship with the assignment of interest (and not, e.g., eligibility to another program which may affect the outcome of interest). If these conditions hold, then an unbiased effect of assignment can be estimated using two-stage regression methods [10] . The challenge lies not in the analysis itself (although such analyses are, typically, inefficient) but in demonstrating that the conditions for having a good instrument are met.

In the case of Bolsa Alimentação in Brazil, a computer error led eligible participants whose names contained nonstandard alphabetical characters to be excluded from the program. Because there are no reasons to believe that these individuals would have had systematically different characteristics to others, the exclusion of individuals was considered “as good as random” (i.e., a true natural experiment based on quasi-random assignment) [9] .

Comparatively few studies in this review used ITS estimation, and we are not aware of any studies in this literature which have been able to draw on sufficiently long time series with longitudinal data for individual units of observation in order for the design to qualify “as good as randomized.” An evaluation of Nepal's Safe Delivery Incentive Programme (SDIP) drew on multiple cohorts of eligible households before and after implementation over a 7-year period [24] . The outcome (neonatal mortality) for each household was available at points in time that could be related to the inception of the program. Unfortunately, comparison group data were not available for nonparticipants, so an analysis of secular trends due to general improvements in maternal and child health care (i.e., not due to SDIP) was not possible. However, the authors were able to implement a regression “placebo test” (sometimes called a “negative control”), in which SDIP treatment was linked to an outcome (use of antenatal care) which was not expected to be affected by the program, the rationale being that the lack of an estimated spike in antenatal care at the time of the expected change in mortality might suggest that these other confounding factors were not at play. But ultimately, due to the lack of comparison group data, the authors themselves note that the study is only able to provide “plausible evidence of an impact” rather than probabilistic evidence (p. 224).

Individual-level DID analyses use participant-level panel data (i.e., information collected in a consistent manner over time for a defined cohort of individuals). The Familias en Accion program in Colombia was evaluated using a DID analysis, where eligible and ineligible administrative clusters were matched initially using propensity scores. The effect of the intervention was estimated as the difference between groups of clusters that were or were not eligible for the intervention, taking into account the propensity scores on which they were matched [25] . DID analysis is only a credible method when we expect unobservable factors which determine outcomes to affect both groups equally over time (the “common trends” assumption). In the absence of common trends across groups, it is not possible to attribute the growth in the outcome to the program using the DID analysis. The problem is that we rarely have multiple period baseline data to compare variation between groups in outcomes over time before implementation, so the assumption is not usually verifiable. In such cases, placebo tests on outcomes which are related to possible confounders, but not the program of interest, can be investigated (see also above). Where multiple period baseline data are available, it may be possible to test for common trends directly and, where common trends in outcome levels are not supported, undertake a “difference-in-difference-in-differences” (DDDs) analysis. In Cambodia, the evaluators used DDD analysis to evaluate the Cambodia Education Sector Support Project, overcoming the observed lack of common trends in preprogram outcomes between beneficiaries and nonbeneficiaries [26] .

As in the case of Attanasio et al. above [25] , difference studies are usually made more credible when combined with methods of statistical matching because such studies are restricted to (or weighted by) individuals and groups with similar probabilities of participation based on observed characteristics—that is, observations “in the region of common support.” However, where panel or multiple time series cohort data are not available, statistical matching methods are often used alone. By contrast with the above examples, a conventional cohort study design was used to evaluate Tekoporã in Paraguay, relying on PSM and propensity weighted regression analysis of beneficiaries and nonbeneficiaries at entry into the cohort to control for confounding [27] . Similarly, for Bolsa Familia in Brazil evaluators applied PSM to cross-sectional (census) data [28] . Variables used to match observations in treatment and comparison should not be determined by program participation and are therefore best collected at baseline. However, this type of analysis alone does not satisfy the criterion of enabling adjustment for unobservable sources of confounding because it cannot rule out confounding of health outcomes data by unmeasured confounding factors, even when participants are well characterized at baseline.

3. Part 2: “quasi-experimental” designs used by health care evaluation researchers

The term “quasi-experimental” is also used by health care evaluation and social science researchers to describe studies in which assignment is nonrandom and influenced by the researchers. At the first appearance, many of the designs seem similar, although they are often labeled differently. Although an assignment rule may be known, it may not be exploitable in the way described above for health system evaluations; for example, quasi-random allocation may be biased because of a lack of concealment, even when the allocation rule is “as good as random.”

Researchers also use more conventional epidemiological designs, sometimes called observational, that exploit naturally occurring variation. Sometimes, the effects of interventions can be estimated in these cohorts using instrumental variables (prescribing preference; surgical volume; geographic variation, distance from health care facility), quantifying the effects of an intervention in a way that is considered to be unbiased [34] , [35] , [36] . Instrumental variable estimation using data from a randomized controlled trial to estimate the effect of treatment in the treated, when there is substantial nonadherence to the allocated intervention, is a particular instance of this approach [37] , [38] .

Nonrandomized study design labels commonly used by health care evaluation researchers include: nonrandomized controlled trial, controlled before-and-after study (CBA), interrupted time series study (ITS; and CITS), prospective, retrospective or historically controlled cohort studies (PCS, RCS and HCS respectively), nested case–control study, case–control study, cross-sectional study, and before-after study. Thumbnail sketches of these study designs are given in Box 2 . In addition, researchers sometimes report findings for uncontrolled cohorts or individuals (“case” series or reports), which only describe outcomes after an intervention [54] ; these are not considered further because these studies do not collect data for an explicit comparator. It should be noted that these sketches are the authors' interpretations of the labels; studies that other researchers describe using these labels may not conform to these descriptions.

Thumbnail sketches of quasi-experimental study designs used by health care evaluation researchers

The designs can have diverse features, despite having the same label. Particular features are often chosen to address the logistical challenges of evaluating particular research questions and settings. Therefore, it is not possible to illustrate them with examples drawn from a single review as in part 1; instead, studies exemplifying each design are cited across a wide range of research questions and settings. The converse also occurs, that is, study design labels are often inconsistently applied. This can present great difficulties when trying to classify studies, for example, to describe eligibility for inclusion in a review. Relying on the study design labels used by primary researchers themselves to describe their studies can lead to serious misclassifications.

For some generic study designs, there are distinct study types. For example, a cohort study can study intervention and comparator groups concurrently, with information about the intervention and comparator collected prospectively (PCS) or retrospectively (RCS), or study one group retrospectively and the other group prospectively (HCS). These different kinds of cohort study are conventionally distinguished according to the time when intervention and comparator groups are formed, in relation to the conception of the study. Some studies are sometimes incorrectly termed PCS, in our view, when data are collected prospectively, for example, for a clinical database, but when definitions of intervention and comparator required for the evaluation are applied retrospectively; in our view, this should be an RCS.

4. Part 3: study design features and their role in disambiguating study design labels

Some of the study designs described in parts 1 and 2 may seem similar, for example, DID and CBA, although they are labeled differently. Some other study design labels, for example, CITS/ITS, are used in both types of literature. In our view, these labels obscure some of the detailed features of the study designs that affect the robustness of causal attribution. Therefore, we have extended the checklist of features to highlight these differences. Where researchers use the same label to describe studies with subtly different features, we do not intend to imply that one or other use is incorrect; we merely wish to point out that studies referred to by the same labels may differ in ways that affect the robustness of an inference about the causal effect of the intervention of interest.

The checklist now includes seven questions ( Table 2 ). The table also sets out our responses for the range of study designs as described in Box 1 , Box 2 . The response “possibly” (P) is prevalent in the table, even given the descriptions in these boxes. We regard this as evidence of the ambiguity/inadequate specificity of the study design labels.

Table 2

Quasi-experimental taxonomy features checklist

Abbreviations: RCT, randomized controlled trial; Q-RCT, quasi-randomized controlled trial; IV, instrumental variable; RD, regression discontinuity; CITS, controlled interrupted time series; ITS, interrupted time series; DID, difference-in-difference; CBA, controlled before-and-after study; NRCT, nonrandomized controlled trial; PCS, prospective cohort study; RCS, retrospective cohort study; HCT, historically controlled study; NCC, nested case–control study; CC, case–control study; XS, cross-sectional study; BA, before-after study; Y, yes; N, no; P, possibly; na, not applicable.

Cells in the table are completed with respect to the thumbnail sketches of the corresponding designs described in Box 1 , Box 2 .

Question 1 is new and addresses the issue of clustering, either by design or through the organizational structure responsible for delivering the intervention ( Box 3 ). This question avoids the need for separate checklists for designs based on assigning individual and clusters. A “yes” response can be given to more than one response item; the different types clustering may both occur in a single study and implicit clustering can occur an individually allocated nonrandomized study.

Clustering in studies evaluating the effects of health system or health care interventions

Clustering is a potentially important consideration in both RCTs and nonrandomized studies. Clusters exist when observations are nested within higher level organizational units or structures for implementing an intervention or data collected; typically, observations within clusters will be more similar with respect to outcomes of interest than observations between clusters. Clustering is a natural consequence of many methods of nonrandomized assignment/designation because of the way in which many interventions are implemented. Analyses of clustered data that do not take clustering into account will tend to overestimate the precision of effect estimates.

Clustering occurs when implementation of an intervention is explicitly at the level of a cluster/organizational unit (as in a cluster-randomized controlled trial, in which each cluster is explicitly allocated to control or intervention). Clustering can also arise implicitly, from naturally occurring hierarchies in the data set being analyzed, that reflect clusters that are intrinsically involved in the delivery of the intervention or comparator. Both explicit and implicit clustering can be present in a single study.

Examples of types of cluster

• • Practitioner (surgeon; therapist, family doctor; teacher; social worker; probation officer; etc.).
• • Organizational unit [general practice, hospital (ward), community care team; school, etc.].
• • Social unit (family unit; network of individuals clustered in some nongeographic network, etc.).
• • Geographic area (health region; city jurisdiction; small electoral district, etc.).

“Explicit” clustering

• • Clustering arising from allocation/formation of groups; clusters can contain only intervention or control observations.

“Implicit” clustering

• • Clustering arising from naturally occurring hierarchies of units of analysis in the data set being analyzed to answer the research question.
• • Clusters can contain intervention and control observations in varying proportions.
• • Factors associated with designation as intervention or control may vary by cluster.

No clustering

• • Designation of an observation as intervention or control is only influenced by the characteristics of the observation (e.g., patient choice to self-medicate with an over-the-counter medication; natural experiment in which allocation of individuals is effectively random, as in the case of Bolsa Alimentação where a computer error led to the allocation to intervention or comparator [31] .)

Question 1 in the checklist distinguishes individual allocation, cluster allocation (explicit clustering), and clustering due to the organizational hierarchy involved in the delivery of the interventions being compared (implicit clustering). Users should respond factually, that is, with respect to the presence of clustering, without making a judgment about the likely importance of clustering (degree of dependence between observations within clusters).

Questions 2–4 are also new, replacing the first question (“Was there a relevant comparison?”) in the original checklist [1] , [2] . These questions are designed to tease apart the nature of the research question and the basis for inferring causality.

Question 2 classifies studies according to the number of times outcome assessments were available. In each case, the response items distinguish whether or not the outcome is assessed in the same or different individuals at different times. Only one response item can be answered “yes.”

Treatment effects can be estimated as changes over time or between groups. Question 3 aims to classify studies according to the parameter being estimated. Response items distinguish changes over time for the same or different individuals. Only one response item can be answered “yes.”

Question 4 asks about the principle through which the primary researchers aimed to control for confounding. Three response items distinguish methods that:

• a. control in principle for any confounding in the design, that is, by randomization, IVE, or regression discontinuity;
• b. control in principle for time invariant unobserved confounding, that is, by comparing differences in outcome from baseline to end of study, using longitudinal/panel data for a constant cohort; or
• c. control for confounding only by known and observed covariates (either by estimating treatment effects in “adjusted” statistical analyses or in the study design by restricting enrollment, matching and/or stratified sampling on known, and observed covariates).

The choice between these items (again, only one can be answered “yes”) is key to understanding the basis for inferring causality.

Questions 5–7 are essentially the same as in the original checklist [1] , [2] . Question 5 asks about how groups (of individuals or clusters) were formed because treatment effects are most frequently estimated from between group comparisons. An additional response option, namely by a forcing variable, has been included to identify credible quasi-experimental studies that use an explicit rule for assignment based on a threshold for a variable measured on a continuous or ordinal scale or in relation to a spatial boundary. When answering “yes” to this item, the review author should also identify the nature of the variable by answering “yes” to another item. Possible assignment rules are identified: the action of researchers, time differences, location differences, health care decision makers/practitioners, policy makers, on the basis of the outcome, or some other process. Other, nonexperimental, study designs should be classified by the method of assignment (same list of variables) but without there being an explicit assignment rule.

Question 6 asks about important features of a study in relation to the timing of their implementation. Studies are classified according to whether three key steps were carried out after the study was designed, namely: acquisition of source data to characterize individuals/clusters before intervention; actions or choices leading to an individual or cluster becoming a member of a group; and the assessment of outcomes. One or more of these items can be answered “yes,” as would be the case for all steps in a conventional RCT.

Question 7 asks about the variables that were measured and available to control for confounding in the analysis. The two broad classes of variables that are important are the identification and collection of potential confounder variables and baseline assessment of the outcome variable(s). The answers to this question will be less important if the researchers of the original study used a method to control for any confounding, that is, used a credible quasi-experimental design.

The health care evaluation community has historically been much more difficult to win around to the potential value of nonrandomized studies to evaluate interventions. We think that the checklist helps to explain why, that is, because designs used in health care evaluation do not often control for unobservables when the study features are examined carefully. To the extent that these features are immutable, the skepticism is justified. However, to the extent that studies may be possible with features that promote the credibility of causal inference, health care evaluation researchers may be missing an opportunity to provide high-quality evidence.

Reflecting on the circumstances of nonrandomized evaluations of health care and health system interventions may provide some insights why these different groups have disagreed about the credibility of effects estimated in quasi-experimental studies. The checklist shows that credible quasi-experimental studies gain credibility from using high-quality longitudinal/panel data; such data characterizing health care are rare, leading to evaluations that “make do” with the data that are available in existing information systems.

The risk of confounding in health care settings is inherently greater because participants' characteristics are fundamental to choices about interventions in usual care; mitigating against this risk requires high-quality clinical data to characterize participants at baseline and, for pharmaco-epidemiological studies about safety, often over time. Important questions about health care for which quasi-experimental methods of evaluation are typically considered are often to do with the outcome of discrete episodes of care, usually binary, rather than long-term outcomes for a cohort of individuals; this can lead to a focus on the invariant nature of the organizations providing the care rather than the varying nature of the individuals receiving care. These contrasts are apparent between, for example: DID studies using panel data to evaluate an intervention such as CCT among individuals with CBA studies of an intervention implemented at an organizational level studying multiple cross-sections of health care episodes; or credible and less credible interrupted time series.

There is a new article in the field of hospital epidemiology which also highlights various features of what it terms as quasi-experimental designs [56] . The list of features appears to be aimed at researchers designing a quasi-experimental study, acting more as a prompt (e.g., “consider options for …”) rather than as a checklist for a researcher appraising a study to communicate clearly to others about the nature of a published study, which is our perspective (e.g., a review author). There is some overlap with our checklist, but the list described also includes several study attributes intended to reduce the risk of bias, for example, blinding. By contrast, we consider that an assessment of the risk of bias in a study is essential and needs to be carried out as a separate task.

5. Conclusion

The primary intention of the checklist is to help review authors to set eligibility criteria for studies to include in a review that relate directly to the intrinsic strength of the studies in inferring causality. The checklist should also illuminate the debate between researchers in different fields about the strength of studies with different features—a debate which has to date been somewhat obscured by the use of different terminology by researchers working in different fields of investigation. Furthermore, where disagreements persist, the checklist should allow researchers to inspect the basis for these differences, for example, the principle through which researchers aimed to control for confounding and shift their attention to clarifying the basis for their respective responses for particular items.

Acknowledgments

Authors' contributions: All three authors collaborated to draw up the extended checklist. G.A.W. prepared the first draft of the paper. H.W. contributed text for Part 1. B.C.R. revised the first draft and created the current structure. All three authors approved submission of the final manuscript.

Funding: B.C.R is supported in part by the U.K. National Institute for Health Research Bristol Cardiovascular Biomedical Research Unit. H.W. is supported by 3ie.

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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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• > 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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