How does a nested case-control within a cohort optimize resource use for rare outcomes?

• A. Cases and matched controls drawn from the same cohort; efficient exposure assessment, reduces cost
• B. Random sample of the cohort
• C. Use the entire cohort
• D. Only use controls

Answer: (A)

In a nested case-control design within a cohort, you identify all cases of the outcome inside the cohort and, for each case, select one or more controls who were at risk at the time the case occurred. This setup uses risk-set sampling, so cases and controls come from the same cohort and share a similar background profile, which helps keep comparisons valid.

The key advantage is efficiency: exposure data are costly to obtain, so you only measure exposure for the cases and a small, representative set of controls rather than the entire cohort. Because the controls come from the same cohort and are selected at the moment of each case, the approach preserves the temporal relationship and yields an unbiased or near-unbiased estimate of the exposure effect, often interpreted as an incidence-rate ratio. This design is particularly powerful when outcomes are rare, since you gain the most information from a small, well-chosen sample instead of entire population data.

Other approaches would either require measuring exposure in many more people (raising costs) or fail to leverage the informative structure of the cohort, making them less efficient for rare outcomes.