In which situation does the odds ratio calculated from a case-control study provide the closest approximation to the relative risk?
- A When the outcome is rare in the source population ✓
- B When controls are recruited from hospitalized patients
- C When the exposure is very common
- D When the confidence interval around the odds ratio is wide
Correct answer: A. When the outcome is rare in the source population
Explanation
When disease incidence is low, conventionally below about 5%, the rare disease assumption holds: the odds of disease approximate its risk, so the exposure odds ratio approaches the relative risk. Hospital-based controls affect representativeness and validity rather than this mathematical property, a wide confidence interval signals imprecision, and a highly prevalent exposure does not restore the risk-odds equivalence.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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