To test whether mobile phone use immediately before driving increases the risk of road traffic accidents, investigators identify accident victims and compare their exposure in the hour before the crash with their own exposure in comparable hours when no crash occurred. This design is called:

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

Explanation

The case-crossover design is built for transient exposures that briefly trigger acute outcomes. Each subject acts as his or her own control, so the hazard period exposure is compared with the same person's exposure during control periods, automatically controlling fixed confounders such as personality and driving skill. Cohort designs need long follow-up and cannot capture momentary exposure windows, and ordinary case-control controls are different persons.

Reference: Modern Epidemiology (Rothman), 4th ed.

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