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Based on this scenario, diagnose whether the researcher's findings describe a main effect of therapy or an interaction effect between therapy and motivation. Justify your diagnosis by explaining how the impact of receiving therapy behaves differently depending on the levels of the patient's motivation.
Case context: A clinical researcher evaluates a study investigating a new therapeutic approach. The study reports that overall, patients who received therapy improved more than those in the control group. However, a closer look at the data shows that this improvement occurred almost exclusively among patients who reported high motivation to change prior to therapy. For patients with low motivation, there was no significant difference in improvement between the therapy group and the control group.
Question: Based on this scenario, diagnose whether the researcher's findings describe a main effect of therapy or an interaction effect between therapy and motivation. Justify your diagnosis by explaining how the impact of receiving therapy behaves differently depending on the levels of the patient's motivation.
Sample answer: The findings describe an interaction effect. While there might appear to be an overall benefit of therapy, the effectiveness of receiving therapy (therapy vs. control) depends entirely on the patient's level of motivation. Specifically, the positive impact of therapy is strong for highly motivated patients but nonexistent for low-motivation patients. Because the impact of one independent variable changes depending on the level of the other, this represents an interaction effect rather than a simple main effect.
Key points:
- Diagnose the scenario as demonstrating an interaction effect.
- State that the effectiveness of therapy (the first independent variable) relies on the level of motivation (the second independent variable).
- Describe that therapy is highly beneficial for motivated patients but has no effect on unmotivated patients.
- Distinguish this pattern from a main effect where therapy would be equally effective regardless of motivation.
Rubric: The response should diagnose the findings as an interaction effect and provide a justification explaining that the effect of the therapy variable (comparing therapy to control) is conditional on the level of the motivation variable (high vs. low).
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Research Methods in Psychology - 4th American Edition @ KPU
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