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9.5: Summary of Quasi-Experimental Research

  • Page ID
    240813
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    Key Takeaways

    • Quasi-experimental research involves the manipulation of an independent variable without the random assignment of participants to conditions or counterbalancing of orders of conditions.
    • There are quasi-experimental designs that compare the scores of participants before and after the treatment without a comparison group; the comparisons are between the timeframes.
    • There are quasi-experimental designs that compare the scores of participants before and after the treatment and a (nonequivalent) comparison group.
    • Quasi-experimental research does not provide initial equivalence between the groups because it does not involve random assignment to conditions or counterbalancing. For this reason, quasi-experimental research has lower internal validity than experimental designs. This means that experiments are stronger evidence that the IV caused changes in the DV than quasi-experimental designs. However, the more that alternative explanations of changes to the outcome variable (DV) other than the intervention group, the more higher the internal validity. Of all of the quasi-experimental designs, those that include a switching replication are highest in internal validity.
    • There are typical issues in quasi-experimental designs that could affect internal validity. These are called threats to internal validity. Additional threats to internal validity can affect experimental designs, as well.

    What's Next?

    Now that you're a little more comfortable with experimental designs and quasi-experimental designs, we're going to learn about slightly more complex designs as they include more than one IV (not just more than one group).

    Exercises
    • Practice: Imagine that two professors decide to test the effect of giving daily quizzes on student performance in a statistics course. They decide that Professor A will give quizzes but Professor B will not. They will then compare the performance of students in their two sections on a common final exam. List five other variables that might differ between the two sections that could affect the results.
    • 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.
      • What kind of quasi-experimental design would this be?
      • For each of the following threats to internal validity for quasi-experimental designs, explain how each of the following might affect the results (or not):
        • History
        • (Re-)Testing
        • Instrumentation
        • Regression to the Mean
        • Attrition/ Mortality
        • Selection
        • Maturation
        • Spontaneous Remission
      • For each of the following threats to internal validity for all designs, explain how each of the following might affect the results (or not):
        • Contamination: Rivalry
        • Contamination: Diffusion of Treatment
        • Contamination: Equalization
        • Experimenter Expectancy Effects
        • Novelty Effects
    • Practice: Some schools have a lab section required for their research methods class. To see if the lab portion of this class improves scores on the class research project's manuscript, an instructor could randomly assign half of the class to stay for the lab section, and the other half of the class could be banned from the lab section of this course. In this scenario, which threats to internal validity might be present? Why?
      • Contamination: Rivalry
      • Contamination: Diffusion of Treatment
      • Contamination: Equalization
      • Experimenter Expectancy Effects
      • Novelty Effects

    This page titled 9.5: Summary of Quasi-Experimental Research is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rajiv S. Jhangiani, I-Chant A. Chiang, Carrie Cuttler, & Dana C. Leighton via source content that was edited to the style and standards of the LibreTexts platform.