Anatomy of the healthy worker effect. A critique of summary statistics employed in occupational epidemiology

The 'healthy worker effect', perhaps more accurately termed the 'active worker effect', has been acknowledged by numerous investigators who have disregarded its significance. The healthy worker effect is expressed by SMRs (standardized mortality ratios) and influenced by the following three factors: selection bias, improved socioeconomic status, and the conventional way of calculating SMRs. An examination of these three factors shows that selection for 'employability' is probably the most significant factor for the healthy worker effect. For example, the shorter the observation, the smaller the SMR, and the stronger the healthy worker effect. Secondly, improved socioeconomic status as a result of employment has been shown to lower mortality. Last, although the SMR has the advantage of estimating relative risk in a small sample, it suffers many methodological pitfalls. It is recommended that at least three parameters be used to summarize mortality experience among the employed: relative risk, attributable risk, and life expectancy.