CONTROLLING FOR TIME-SINCE-HIRE IN OCCUPATIONAL STUDIES USING INTERNAL COMPARISONS AND CUMULATIVE EXPOSURE

Employees within an occupational cohort may demonstrate a more favorab le mortality experience while maintaining employment than those who le ave employment. At the same time, they may experience an apparent decl ine in health with time-since-hire. The time-since-hire effect may occ ur independently of exposure but may nevertheless result in groups cat egorized by cumulative exposure that are not comparable. Controlling f or time-since-hire appears to solve this problem. To quantify the empi rical bias in estimates of exposure effect due to confounding from tim e-since-hire, we analyzed two occupational cohorts using Poisson regre ssion with and without ad adjustment for time-since-hire or time-since -start-of-follow-up. In a cohort exposed to airborne arsenic, a strong dose-response relation with respiratory cancer mortality had been est ablished: In a cohort exposed to external, penetrating ionizing radiat ion, a weak and controversial dose-response relation had been reported . The parameter estimates relating exposure to disease from the models that explicitly adjusted for time-since-hire or time since-start-of-f ollow-up are within 10% of the estimates from models that did not. It appears, from this empirical analysis of two datasets, that occupation al studies may not need to adjust explicitly for such time-related fac tors as time-since-hire or time-since-start-of-follow-up if these are implicitly controlled through other variables in the model.