Confounding by time since hire in internal comparisons of cumulative exposure in occupational cohort studies.

We use a simple, empirical model to describe the healthy worker effect mortality pattern. Under this simple model, internal comparisons of risk with increasing cumulative exposure will tend to be biased away from the null because of the healthy worker effect. We illustrate the potential magnitude of the bias in a simple situation and show that controlling for time since hire, by means of standard epidemiologic methods, eliminates the bias. Time since hire also is a concern of occupational epidemiologists because of the issue of induction time; sufficient time may not have elapsed among recently hired workers for an exposure to manifest its effect on disease occurrence. Provision for an adequate induction period can be addressed, like the concern raised in this paper, by restricting the analysis to workers first employed many years before the start of the follow-up period.