Occupational cohort studies: the influence of design characteristics on the healthy worker effect

A cross-sectional analysis of the relation between the estimated healthy worker effect (HWE), as measured by a total Standardized Mortality Ratio (SMR) less than 100, and relevant design characteristics in 270 published retrospective occupational cohort studies is presented. The majority of the reviewed studies showed a HWE, varying in SMR from 50 to 99 (mean: 84). The estimated HWE seemed to influence the final outcomes of the studies to a great extent. A tendency for a positive relation between the study size in terms of the number of exposed workers, total number of person-years of follow-up, and the HWE emerged. Studies with a comparatively short follow-up period had an increased chance of resulting in a HWE. Cross-sectional cohorts did not show a stronger HWE than open cohorts. Studies of chemical exposures revealed a fivefold excess of having a HWE compared with other studies.