Which study design allows a subject to serve as their own control?

• A. Prospective cohort
• B. Retrospective cohort
• C. Case-cohort
• D. Case-crossover

Answer: (D)

The design that lets a subject serve as their own control uses comparisons within the same person across different time periods. In a case-crossover study, exposure in a brief window just before the acute event (the hazard period) is compared to exposure in one or more other time windows for the same individual. This within-person comparison inherently controls for fixed characteristics like genetics or chronic health status, since those factors don’t change over the short time frame being studied. It’s especially useful for identifying transient triggers of acute events, where the exposure fluctuates over time. Other designs compare different individuals (between-person comparisons), so they don’t provide the same within-person control.