A case-control study finds coffee drinking associated with lung cancer, odds ratio 1.8. Coffee drinkers in this sample also smoke substantially more than non-drinkers, and smoking is a strong independent cause of lung cancer. Smoking is best described as a:

• A Confounder, because it is associated with the exposure, independently affects the outcome, and is not on the causal pathway between coffee and lung cancer ✓
• B Mediator, because smoking lies on the causal pathway between coffee and lung cancer
• C Effect modifier, because the effect of coffee differs across strata of smoking
• D Collider, because it is caused by both coffee drinking and lung cancer

Explanation

A confounder must fulfil three conditions: association with the exposure in the source population, being an independent risk factor for the outcome, and lying outside the causal pathway between exposure and outcome. Smoking satisfies all three here, and stratifying or adjusting for smoking should abolish or shrink the apparent coffee effect. A mediator lies on the pathway, an effect modifier requires the association to genuinely differ across strata, and a collider is a common effect of two variables.

Reference: Gordis Epidemiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.