A student calculates an odds ratio of 6.2 for alcohol use and esophageal cancer in a hospital-based case-control study. Her teacher notes that this figure can be accepted as a close approximation of the true relative risk only under a specific condition, which is:

• A Exposure prevalence in controls matches that of the general population
• B Cases and controls are recruited from the same hospital
• C The confidence interval excludes the null value
• D The outcome disease is rare in the source population ✓

Explanation

The odds ratio converges on the relative risk only when the disease is uncommon, conventionally below about 10% cumulative incidence, because then the odds of disease approximate its probability among both exposed and unexposed groups. Esophageal cancer satisfies this rarity, so the OR of 6.2 closely estimates the RR. Hospital matching and statistical significance affect validity and precision respectively, but neither permits the OR to RR substitution.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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