A study finds a higher incidence of hypertension in coffee drinkers. Further analysis reveals that coffee drinkers also smoke more than non-coffee drinkers, and smoking is an independent risk factor for hypertension. Smoking in this scenario is best described as:

• A An effect modifier
• B An intermediate variable
• C Collider bias
• D A confounder ✓

Explanation

A confounder is a variable that: (1) is associated with the exposure (coffee drinking), (2) is independently associated with the outcome (hypertension), and (3) is not on the causal pathway between exposure and outcome. Smoking fulfills all three criteria here — it distorts the apparent coffee-hypertension relationship. An effect modifier changes the magnitude of the exposure-outcome association in different strata. An intermediate variable lies on the causal pathway (controlling for it would be inappropriate).

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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