Smith, Glass, and Miller's 1980 Psychotherapy Effectiveness Study

What is the primary purpose of adding a control group to a pretest-posttest design?

To properly rule out threats to internal validity in a pretest-posttest design, the control group must be shielded from the external events and passage of time experienced by the treatment group.

A researcher is studying the effectiveness of a new therapy for depression using a pretest-posttest design with a control group. Match each internal validity threat to the specific way the inclusion of a control group helps rule it out in this scenario.

A researcher using a pretest-posttest design with a control group observes that the treatment group's scores improved. Arrange the logical steps the researcher must follow to analyze these results and isolate the unique effect of the treatment from alternative explanations like maturation or history.

In a pretest-posttest design, which of the following is experienced by the control group?

In a pretest-posttest control group design, each component of the control group's experience helps rule out a specific threat to internal validity. Match each action taken with the control group to the alternative explanation it is designed to address.

If a researcher observes that both the treatment group and the control group in a pretest-posttest design show identical levels of improvement, the researcher must evaluate the treatment as having _____ actual effect on the outcome.

A researcher tests a new memory training program using a pretest-posttest design with a control group. At the end of the study, both the treatment group and the control group show similar improvements in memory scores from pretest to posttest. Applying the logic of pretest-posttest control group designs, the researcher should conclude that the memory training program caused the improvement in the treatment group.

A researcher studying the effect of a study-skills workshop on exam performance uses a pretest-posttest design with a control group. After the intervention, exam scores improved for students in both the treatment group and the control group. Analyzing these results, the researcher would identify the control group's improvement as most likely reflecting a _____ threat to internal validity, because repeated exposure to similar exams—rather than the workshop itself—may account for the score gains.

A critic is evaluating whether a researcher's conclusion—that a 10-week mindfulness program caused a reduction in anxiety—is justified. The researcher used a pretest-posttest design with a control group. Arrange the following steps in the order the critic should follow to render this evaluative judgment.

List the six specific threats to internal validity that can be effectively ruled out by adding a control group to a pretest-posttest design. In addition, describe how the control group is treated in comparison to the treatment group.

Explain why comparing the pretest-to-posttest changes between the treatment group and the control group allows the researcher to comprehend and isolate the actual effect of the memory-training program from alternative explanations.

A school psychologist tests a new reading app. The treatment group (using the app) improves their reading speed by $20$ words per minute from pretest to posttest, while the control group (using standard books) also improves by $20$ words per minute. Apply your understanding of pretest-posttest control group designs to state the conclusion the psychologist should draw about the app's effectiveness and why.