To test whether mobile phone use immediately preceding driving raises the risk of road traffic accidents, investigators enroll drivers involved in crashes and compare each driver's own phone usage in the hazard interval just before the crash against their usage during identical time windows on previous days without a crash. Which design has been used?

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

Explanation

The case-crossover design uses each subject as his or her own control, comparing transient exposure immediately before the event with the same individual's exposure during control periods. It suits brief, intermittent exposures with short-term effects such as phone use, physical exertion or certain drugs before sudden events. Because each participant serves as the comparison, fixed personal confounders cancel out. Nested case-control designs compare different individuals within a cohort, and case-cohort designs sample a subcohort as the comparator group.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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