To test whether vigorous physical activity acutely triggers myocardial infarction, researchers enrol patients who have just suffered an MI and compare their activity levels in the hours immediately preceding the attack with their own usual activity levels at the same time of day on previous days. Each patient thus serves as his own control. This design is:

• A Case-crossover study ✓
• B Case-cohort study
• C Cross-over randomised controlled trial
• D Retrospective panel study

Explanation

The case-crossover design was developed by Maclure for studying transient exposures that trigger acute events. Each case contributes paired exposure windows, the hazard period just before the event and one or more control periods, so every subject acts as his own control and fixed personal confounders cancel out. It suits intermittent short-lived exposures such as exercise, anger, sexual activity or mobile phone use before road accidents.

Reference: Rothman, Greenland and Lash, Modern Epidemiology, 3rd ed.

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