A case-case study is a variant of case-crossover that studies the temporal association between time-varying exposure and outcome only in which subjects?

• A. All individuals
• B. Controls
• C. Cases
• D. Both cases and controls

Answer: (C)

In a case-case design, the temporal link between a time-varying exposure and the onset of the outcome is assessed by comparing exposure within the same person across different time windows around the event. The analysis uses only those individuals who have experienced the outcome, so the subjects studied are the cases themselves. This within-person comparison controls for fixed characteristics and isolates the potential trigger effect of the exposure. Controls are not included in this design because the goal is to examine the temporal pattern of exposure leading up to the event within cases.