How do I conduct a quasi-experiment?

• Assign entire groups to treatment vs. control conditions

An essential feature of an experiment is that the researcher compares at least two conditions. One group receives a "treatment," and the other does not. In quasi-experimental designs, rather than randomly assigning individual participants to treatment and control conditions, we might assign an entire group to receive a treatment and withhold the treatment from another group.

For example, we might test the hypothesis that students who are allowed to choose the type of assignments they complete in a course perform better than students who are not given a choice. The independent variable is whether students are allowed choice. The dependent variable could be their final grade for the course.

You may see that it wouldn’t be fair to allow some students in a class to choose their assignments and give other students in the class no choice. Therefore, we might manipulate the independent variable using two different sections of the same course. That is, students in one section of the course would be allowed to make choices and students in another section would not make choices. We would hold constant that students have to do the same number of assignments.

Although this experiment includes an independent variable (choice) and a dependent variable (grade), we have no control over many aspects of this experiment. Most importantly, students in the two sections are likely to be different. Suppose one section meets at 8:00 a.m. and another section meets at 2:00 p.m. Students who enroll in an 8:00 class are likely to be different from students who select a 2:00 class. In addition, class discussions may differ during the academic term, and the instructor may cover slightly different material. All of these potential variables may influence the outcome–students’ final grade in the course.

Quasi-experiments provide some information about variables, but the cause-and-effect relationship between choosing assignments and grades may not be clear at the end of the study. Suppose students who are allowed to choose their assignments earn higher grades than students who are not allowed a choice. Can we confidently say that our independent variable, assignment choice, caused this difference in grades? Researchers who conduct quasi-experiments often face difficult decisions about whether other variables, such as time of day or material covered in the class, could have caused the different grade outcomes.

Clinical psychologists use quasi-experimental designs to answer some of the questions they are interested in. For example, if a clinical psychologist is interested in preventing aggressive behavior in school classrooms, she or he may be required to assign whole classrooms of children to a treatment condition or a control condition in order to evaluate the effects of an intervention to prevent aggressive behavior.

Thus, if in your research question you seek to examine the causal effect of an independent variable on a dependent variable, but you cannot control other important variables in the research, you should use a quasi-experimental design.