What is the healthy worker effect and how can it bias cohort results?

• A. Non-employed individuals are healthier than workers, biasing results toward null.
• B. Employed individuals tend to be healthier than the general population, potentially underestimating risk when using the general population as a reference.
• C. Healthy workers have higher risk of disease.
• D. The effect applies only to case-control studies.

Answer: (B)

The healthy worker effect is a selection bias in occupational studies: people who are employed tend to be healthier than the general population, because illness can keep people out of work. When a cohort study compares workers to the general population, this healthier baseline among workers makes them appear at lower risk for disease than they would be if both groups had the same health status. So, even if the exposure increases risk, the overall association can look weaker than it truly is—bias toward the null. This bias arises from who stays in employment (healthier individuals) and who leaves or never enters the workforce (often those with illness), not from the exposure itself. It does not imply that healthy workers have higher disease risk, and it can affect cohort studies just as it can affect other study designs.