In a case-control study of a disease with low prevalence, the odds ratio closely approximates the relative risk that a cohort study would have produced. This approximation holds primarily because:
- A When disease is rare, the odds of disease in both exposed and unexposed groups approximate the risk of disease ✓
- B Controls are sampled from the general population
- C Case-control studies eliminate confounding by design
- D Matching ensures exchangeability of cases and controls
Correct answer: A. When disease is rare, the odds of disease in both exposed and unexposed groups approximate the risk of disease
Explanation
Risk equals odds divided by one plus odds. When disease is rare, the probability of disease is small enough that one plus the odds is close to one, so the odds of disease approximate its risk in both exposed and unexposed groups and the exposure odds ratio approximates the risk ratio. Sampling method, matching and confounding control do not create this mathematical equivalence.
Reference: Gordis Epidemiology, 6th ed.
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