A cohort study reports a crude relative risk of 2.5 for coffee intake and pancreatic cancer. When the data are stratified by smoking status, the relative risk is 1.0 among non-smokers and 1.1 among smokers, with tight confidence intervals in each stratum. Smoking is acting as a:
- A Effect modifier
- B Confounder ✓
- C Intermediate variable in the causal chain
- D Independent causal factor with no distortion
Correct answer: B. Confounder
Explanation
When the stratum-specific estimates are similar to each other but differ markedly from the crude estimate, the third variable distorts the true association and is a confounder; adjustment moves the estimate toward the stratum-specific values. Effect modification is diagnosed when stratum-specific estimates differ substantially from each other, indicating the association genuinely varies across levels of the third variable. An intermediate variable lies on the causal pathway and should not be adjusted for.
Reference: Modern Epidemiology (Rothman), 4th ed.
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