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A researcher tests two pain medications (A and B) using a crossover design. 35 patients each receive both medications in random order, separated by a 2-week washout period:
- Phase 1: Medication A or B (randomized), pain relief measured
- Weeks 3–4: Washout (no medication)
- Phase 2: The other medication, pain relief measured
Results show Medication B produces significantly better relief than A (). However, post-hoc analysis reveals that patients who received Medication A first showed sustained improvement even during the washout period and into Phase 2, while patients who received B first did not. The Phase 2 results are confounded by order/carryover effects.
Which statement best explains the threat to validity?
A researcher tests two pain medications (A and B) using a crossover design. 35 patients each receive both medications in random order, separated by a 2-week washout period:
- Phase 1: Medication A or B (randomized), pain relief measured
- Weeks 3–4: Washout (no medication)
- Phase 2: The other medication, pain relief measured
Results show Medication B produces significantly better relief than A (). However, post-hoc analysis reveals that patients who received Medication A first showed sustained improvement even during the washout period and into Phase 2, while patients who received B first did not. The Phase 2 results are confounded by order/carryover effects.
Which statement best explains the threat to validity?