Reducing healthy worker survivor bias by restricting date of hire in a cohort study of Vermont granite workers

Objective: To explore the healthy worker survivor effect (HWSE) in a study of Vermont granite workers by distinguishing "prevalent'' from "incident'' hires based on date of hire before or after the start of follow-up. Methods: Records of workers between 1950 and 1982 were obtained from a medical surveillance programme. Proportional hazards models were used to model the association between silica exposure and lung cancer mortality, with penalised splines used to smooth the exposure-response relationship. A sensitivity analysis compared results between the original cohort and subcohorts defined by restricting date of hire to include varying proportions of prevalent hires. Results: Restricting to incident hires reduced the 213 cases by 74% and decreased the exposure range. The maximum mortality rate ratio (MRR) was close to twofold in all subcohorts. However, the exposure at which the maximum MRR was achieved decreased from 4.0 to 0.6 mg-year/m(3) as the proportion of prevalent hires decreased from 50% in the original cohort to 0% in the subcohort of incident hires. Conclusion: Despite loss in power and restricted exposure range, decreasing the relative proportion of prevalent to incident hires reduced HWSE bias, resulting in stronger evidence for a dose-response between silica exposure and lung cancer mortality.