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

Ellen A Eisen ^1*, Katie Applebaum ¹ and Elizabeth J Malloy ²

¹ Harvard School of Public Health, United States
² American University, United States

^* To whom correspondence should be addressed. E-mail: eeisen{at}hsph.harvard.edu.

Accepted 30 March 2007

Abstract

Objective: 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 program. Proportional hazards models were used to model the association between silica exposure and lung cancer mortality, with penalized splines used to smooth the exposure-response relationship. A sensitivity analysis compared results between the original cohort and sub-cohorts 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 2-fold in all sub-cohorts. However, the exposure at which the maximum MRR was achieved decreased from 4.0 to 0.6 mg-yr/m3 as the proportion of prevalent hires decreased from 50% in the original cohort to 0% in the sub-cohort 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.

Keywords: healthy worker effect, inception cohort, incident hire, prevalent hire, selection bias

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