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

The case-crossover study design is unique in that it allows subjects to serve as their own controls. In this type of study, researchers focus on events that occur at unique points in time, particularly situations where the exposure is transient, like an acute event. Participants are compared to themselves at different time periods — one when they were exposed to a certain risk factor and another when they were not.

This approach mitigates variability among participants that may confound results. Since each participant acts as their own control, it eliminates the impact of unmeasured confounding variables that could differ between subjects. This design is particularly useful in the study of acute outcomes related to transient exposures, such as the effect of a particular medication or substance on health outcomes like heart attacks or asthma attacks.

In contrast, the other study designs—prospective cohort, retrospective cohort, and case-control studies—compare different individuals or groups, where one group is exposed to a certain risk factor while the other is not, making the direct control comparison less effective without adjusting for individual differences.