Within an established cohort of 20,000 adults followed for incident diabetes, investigators identify all 300 incident cases and then select a sample of non-diabetic members who were under observation when each case occurred, measuring an expensive biomarker stored at baseline only in these selected subjects. This design is called:

• A Case-crossover study
• B Nested case-control study ✓
• C Historical prospective cohort study
• D Case-cohort study

Explanation

Cases and controls are both drawn from inside a defined cohort, and controls are sampled from members still at risk at the time each case arises, which defines a nested (incident-density-sampled) case-control study. It allows costly assays on a fraction of the cohort while retaining temporal sequence from prospectively stored samples. A historical cohort would classify everyone by baseline exposure and follow all for outcomes, and a case-cohort uses a single random subcohort as the comparison group rather than risk-set sampling.

Reference: Gordis Epidemiology, 6th ed.

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