A researcher has an existing cohort of 10,000 factory employees followed since 2010. She now identifies all workers who developed bladder cancer during follow-up and selects four cancer-free workers per case, matched on calendar time and duration of follow-up, from those still under observation at the time each case was diagnosed. Which study design has she employed?

• A Case-cohort study
• B Retrospective cohort study
• C Nested case-control study ✓
• D Cumulative incidence case-control study

Explanation

Sampling controls from the members of an established cohort who are still at risk at the time each case occurs defines a nested (hybrid) case-control design. It retains the temporal logic of a cohort while allowing expensive exposure measurements only on the sampled subjects. C case-cohort study differs because its subcohort of controls is selected once at baseline regardless of whether they remain at risk when each case arises.

Reference: Gordis Epidemiology, 6th ed.

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