An emergency physician studies whether brief strenuous exertion can trigger acute myocardial infarction. For each patient admitted with MI, she compares his physical activity level in the 60 minutes preceding symptom onset with his usual activity level in matched earlier time windows. Each patient thus serves as his own control. This design is a:

• A Cross-over randomized trial
• B Case-control study with neighborhood controls
• C Retrospective cohort study
• D Case-crossover study ✓

Explanation

The case-crossover design suits transient exposures with abrupt hypothesized effects: exposure immediately before the event (hazard period) is compared with the same subject's exposure during earlier reference periods, giving perfect self-matching for stable personal characteristics. A cross-over trial requires deliberate randomization to treatment sequences, and a conventional case-control study uses separate individuals as controls.

Reference: Gordis Epidemiology, 6th ed.

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