A cohort study evaluates the association between oral contraceptive use and deep vein thrombosis. The relative risk is 3.0 in women over 35 years of age and 1.5 in women under 35 years of age. The difference in the strength of the association across age groups is an example of:
- A Confounding
- B Effect modification ✓
- C Selection bias
- D Random error
Correct answer: B. Effect modification
Explanation
Effect modification occurs when the strength of the association between an exposure and an outcome varies across levels of a third variable, known as an effect modifier. Here, age modifies the effect of oral contraceptives on thrombosis risk. Confounding differs because it represents a mixing of effects where a third variable is associated with both the exposure and the outcome, distorting the true relationship rather than showing a genuine difference in effect across strata.
Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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