Controlling for time-since-hire in occupational studies using internal comparisons and cumulative exposure

Employees within an occupational cohort may demonstrate a more favorable mortality experience while maintaining employment than those who leave employment. At the same time, they may experience an apparent decline in health with time-since-hire. The time-since-hire effect may occur independently of exposure but may nevertheless result in groups categorized by cumulative exposure that are not comparable. Controlling for time-since-hire appears to solve this problem. To quantify the empirical bias in estimates of exposure effect due to confounding from time-since-hire, we analyzed two occupational cohorts using Poisson regression with and without 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 established. In a cohort exposed to external, penetrating ionizing radiation, 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-follow-up are within 10% of the estimates from models that did not. It appears, from this empirical analysis of two datasets, that occupational studies may not need to adjust explicitly for such time-related factors as time-since-hire or time-since-start-of-follow-up if these are implicitly controlled through other variables in the model.