What is the benefit of a nested case-control design within a cohort?

• A. Efficient exposure assessment for rare outcomes
• B. Increases cost
• C. It uses the entire cohort
• D. It is irrelevant to rare outcomes

Answer: (A)

The benefit of a nested case-control design within a cohort is efficiency in exposure assessment, especially when the outcome is rare. By measuring exposures only for the cases and a subset of controls drawn from those at risk when each case occurs, you can obtain comparable estimates to a full cohort study but with far less data collection and cost. This approach keeps the exposure data tied to the temporal sequence (exposure before outcome) and allows valid analysis using methods like conditional logistic regression, effectively approximating the cohort’s rate or risk estimates while dramatically reducing effort. It’s not about increasing costs, it doesn’t require evaluating the entire cohort, and it’s particularly advantageous for rare outcomes because sampling saves resources without sacrificing validity.