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Case-control study — USMLE Step 3 MCQ

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HardPreventive Medicine, Epidemiology, BiostatisticsCase-control studyUSMLE Step 3

A case-control study examines prior proton-pump-inhibitor exposure. Of 300 cases, 60% were exposed; of 600 controls, 30% were exposed. Which calculation and interpretation are correct?

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Correct answer: A — The odds ratio is 3.5; cases had 3.5 times the odds of prior exposure compared with controls

The best answer is “The odds ratio is 3.5; cases had 3.5 times the odds of prior exposure compared with controls”. The percentages first give 180 exposed and 120 unexposed cases, and 180 exposed and 420 unexposed controls. The cross-product ratio is (180×420)/(120×180)=3.5. Because sampling was conditioned on outcome, this is an exposure odds comparison; the data do not directly estimate incidence, risk ratios, or attributable risk. For a 2-by-2 case-control table, the odds ratio is ad/bc. The alternatives use a different threshold, timing, population, or intervention and therefore do not match the clinical facts presented.

Reference: CDC: Principles of Epidemiology, Odds Ratio: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section5.html