A case-control study of a rare occupational cancer reports an odds ratio (OR) of 3.2. In this scenario, the OR is considered a good approximation of the relative risk (RR) primarily because:

• A Case-control studies always estimate RR directly
• B The cases were selected from incident cases (not prevalent cases)
• C The disease is rare (prevalence below 10%), satisfying the rare disease assumption ✓
• D The control series is matched to cases for all confounders

Explanation

The odds ratio approximates the relative risk when the disease is rare (prevalence < 10%) — this is called the rare disease assumption. When disease prevalence is low, the odds of disease approximate the probability of disease in each exposure group, making OR ≈ RR. Selection of incident rather than prevalent cases reduces Neyman bias but is not the basis for the OR-RR approximation. Controls being matched reduces confounding but does not affect whether OR ≈ RR.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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