A cohort study finds that chemical plant workers have significantly lower all-cause mortality compared to the general population, despite known exposure to toxic substances. This finding is best explained by:

• A Healthy worker effect ✓
• B Recall bias
• C Lead time bias
• D Berkson's bias

Explanation

The healthy worker effect is a selection bias where employed populations exhibit lower mortality than the general population because the general population includes individuals too ill to work. This biases occupational cohort studies toward finding apparent protective effects of employment. Lead time bias arises in screening studies. Recall bias is an information bias in case-control studies. Berkson's bias is a hospital-based selection bias.

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

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