## Understanding Odds Ratio in Case-Control Studies **Key Point:** In a case-control study, the odds ratio (OR) estimates the odds of *exposure* among cases versus controls — NOT the risk of disease in the exposed group. ### Why OR ≠ Risk Ratio in Case-Control Design Case-control studies are retrospective: they start with diseased (cases) and non-diseased (controls) individuals and look *backward* to exposure history. Therefore: - **Odds Ratio (OR)** = $\frac{\text{Odds of exposure in cases}}{\text{Odds of exposure in controls}}$ - This answers: "How much more likely are cases to have been exposed?" - OR **approximates** relative risk (RR) only when disease is rare (<10% in the population) ### Correct Interpretation of OR = 3.5 **High-Yield:** The OR of 3.5 means women *with VTE* are 3.5 times more likely to report prior OCP use than women *without VTE*. This is the natural direction of inference in a case-control study. ### Why Other Options Are Incorrect | Option | Why Wrong | |--------|----------| | Option 0 | Confuses OR with RR; suggests prospective risk, which case-control cannot measure directly | | Option 1 | Again conflates OR with RR; RR is calculated in cohort studies | | Option 3 | Describes attributable risk or absolute risk difference; OR is a *relative* measure, not absolute | **Clinical Pearl:** When reporting case-control findings to clinicians, always clarify: "Cases were 3.5 times more likely to have been exposed" — not "exposure increases risk 3.5-fold" (which would be RR language). **Warning:** A common NEET PG trap is presenting an OR and asking for RR interpretation. The two are NOT interchangeable unless disease prevalence is <10%.
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