A case-control study reports an odds ratio of 4.0 for the association between alcohol consumption and esophageal cancer. Under which condition does this odds ratio best approximate the relative risk that would be obtained from a cohort study?
- A When the outcome under study is rare in the general population ✓
- B When the exposure prevalence in the population is below 50%
- C When controls are selected from the hospital inpatient pool
- D When the confidence interval of the odds ratio excludes unity
Correct answer: A. When the outcome under study is rare in the general population
Explanation
In a case-control study the denominator of risk cannot be computed because sampling is based on outcome status. The rare disease assumption states that when disease incidence is low, the odds of disease approximate the risk of disease, so OR approximates RR. Esophageal cancer qualifies as a rare outcome. Exposure prevalence and hospital-based control selection affect validity but do not govern the OR-RR approximation.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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