To test whether mobile phone use while driving immediately increases the risk of a road traffic crash, a researcher compares each driver's phone use in the hazard interval just before the crash with phone use by the same driver during earlier control intervals. This design is:

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

Explanation

The case-crossover design, introduced by Maclure, is built for transient exposures with immediate effects. Each case serves as his own control, comparing exposure in the window just before the event with exposure in earlier windows, which automatically controls for stable personal confounders. A cross-over trial is an experimental design with planned treatment sequences, and a retrospective cohort requires a defined exposed and unexposed population followed over time.

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

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