Based on the provided context, identify the major limitation of the psychologist's research design in terms of measurement frequency. Then, explain how transitioning to an interrupted time-series design would address this limitation and why it would lead to a more valid conclusion.
Case context: A clinical psychologist wants to test whether a new weekly therapy session reduces self-reported stress levels in college students. She measures students' stress levels once at the beginning of Week 7 (pretest), administers the therapy session, and measures their stress levels again at the beginning of Week 8 (posttest). She observes a drop in stress scores and concludes the therapy session was highly effective. However, historical data shows that students' stress levels naturally fluctuate significantly from week to week depending on their assignment schedules.
Question: Based on the provided context, identify the major limitation of the psychologist's research design in terms of measurement frequency. Then, explain how transitioning to an interrupted time-series design would address this limitation and why it would lead to a more valid conclusion.
Sample answer: The psychologist's simple pretest-posttest design only measures stress once before and once after the treatment, making it impossible to determine if the drop in stress is a true treatment effect or just a normal weekly fluctuation. By transitioning to an interrupted time-series design, the psychologist would collect multiple measurements of stress both before and after the therapy. This establishes a baseline trend, allowing her to determine whether the drop at Week 8 is a genuine change caused by the therapy or merely part of the normal week-to-week variation.
Key points:
- Diagnoses that single pretest-posttest measurements cannot account for normal week-to-week stress fluctuations.
- Proposes transitioning to an interrupted time-series design by collecting multiple measurements before and after the therapy.
- Explains that multiple measurements establish a baseline trend to rule out normal variation.
- Concludes that the baseline trend allows for a more valid determination of treatment effectiveness.
Rubric: To earn full credit, the student must: 1) Identify that a single pretest and posttest measurement cannot rule out normal week-to-week variation. 2) Explain that an interrupted time-series design requires collecting multiple measurements before and after the therapy. 3) Describe how these multiple measurements establish a baseline trend to isolate the therapy's true effect from normal ongoing fluctuations.
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