Bradford Hill's criterion of 'biological gradient' (dose-response) is demonstrated in a study showing increasing lung cancer risk with increasing pack-years of smoking. Which of the following is MOST accurately described as a separate Bradford Hill criterion that strengthens causal inference?

• A The association is found only in retrospective studies
• B The P-value is less than 0.001
• C The effect is seen only in males
• D The association is consistent across multiple populations and designs ✓

Explanation

Consistency — the association being replicated across different populations, times, and study designs — is a distinct Bradford Hill criterion that strengthens causal inference alongside biological gradient. Statistical significance (p-value) is not a Bradford Hill criterion. Restriction to one sex would relate to specificity, but consistency across populations is the best answer here.

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

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