A study measures the association between coffee consumption and atrial fibrillation. Researchers find an OR of 1.4 (95% CI 1.1–1.8). A reviewer notes that coffee consumption is correlated with smoking, which is the true risk factor. Adjustment for smoking attenuates the OR to 1.0. The original finding illustrates:
- A Effect modification
- B Interaction
- C Confounding ✓
- D Reverse causation
Correct answer: C. Confounding
Explanation
Confounding occurs when a third variable (confounder — here smoking) is associated with both the exposure (coffee) and the outcome (AF), and lies outside the causal pathway between them. After adjustment, the association disappears, confirming confounding rather than a true causal link. Effect modification (interaction) would mean the OR differs significantly between strata of the third variable and is not 'removed' by adjustment — it must be reported separately for each stratum. Reverse causation implies the outcome causes the exposure.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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