An occupational cohort of factory workers shows lower total mortality than the general population, even though the factory clearly exposes workers to a lung carcinogen. Which phenomenon explains this paradox?

• A Neyman's bias
• B Hawthorne effect
• C Ecological fallacy
• D Healthy worker effect ✓

Explanation

Employed populations are systematically healthier than the general population because serious illness excludes people from the workforce, so comparisons against general-population rates are biased downward and may mask true occupational hazards. This is the healthy worker effect, a form of selection bias specific to occupational studies. Neyman's bias concerns survival-dependent entry into case-control studies, the Hawthorne effect describes behaviour change under observation, and ecological fallacy is erroneous inference from group-level data.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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