What is a nested cohort (nested case-control) study?

• A. A cohort drawn from an existing cohort; cases and controls selected from within the cohort
• B. A separate cohort studied in parallel
• C. A cross-sectional snapshot within a cohort
• D. A randomized trial within a cohort

Answer: (A)

A nested case-control study is a case-control study conducted within an existing cohort, with both cases and controls drawn from that same cohort. In this design, you start with a defined group that has been followed over time, identify the individuals who develop the outcome (cases), and then select controls from those cohort members who have not yet developed the outcome at the time each case occurs. This preserves the temporal sequence between exposure and outcome and is efficient for exposure assessment because you only measure or use detailed exposure data for the cases and a subset of controls drawn from the same population. The nested approach keeps everything inside the original cohort, rather than using a separate, external control group.