To test whether heavy physical exertion acts as a trigger for acute myocardial infarction, researchers enrol patients who have just suffered an MI and compare their exertion levels in the hour before symptom onset with their own exertion levels at the same hour on previous days. Each patient thus serves as his own control. This design is called:

• A Nested case-control study
• B Case-cohort study
• C Case-crossover study ✓
• D Retrospective cohort study

Explanation

The case-crossover design compares transient exposure immediately preceding the event (hazard period) with the same individual's usual exposure (control periods), making it ideal for brief intermittent triggers such as exertion, sexual activity or air pollution peaks. Self-matching automatically eliminates fixed confounders like genotype and personality. A nested case-control study instead samples controls from the risk set within a defined cohort, which this design does not do.

Reference: Modern Epidemiology, Rothman, 4th ed.

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