Piecewise exponential models to assess the influence of job-specific experience on the hazard of acute injury for hourly factory workers

Abstract

BackgroundAn inverse relationship between experience and risk of injury has been observed in many occupations. Due to statistical challenges, however, it has been difficult to characterize the role of experience on the hazard of injury. In particular, because the time observed up to injury is equivalent to the amount of experience accumulated, the baseline hazard of injury becomes the main parameter of interest, excluding Cox proportional hazards models as applicable methods for consideration.MethodsUsing a data set of 81,301 hourly production workers of a global aluminum company at 207 US facilities, we compared competing parametric models for the baseline hazard to assess whether experience affected the hazard of injury at hire and after later job changes. Specific models considered included the exponential, Weibull, and two (a hypothesis-driven and a data-driven) two-piece exponential models to formally test the null hypothesis that experience does not impact the hazard of injury.ResultsWe highlighted the advantages of our comparative approach and the interpretability of our selected model: a two-piece exponential model that allowed the baseline hazard of injury to change with experience. Our findings suggested a 30% increase in the hazard in the first year after job initiation and/or change.ConclusionsPiecewise exponential models may be particularly useful in modeling risk of injury as a function of experience and have the additional benefit of interpretability over other similarly flexible models.

Contribution of health status and prevalent chronic disease to individual risk for workplace injury in the manufacturing environment

“…Further, as the hazard of injury decreases after 1 year on the job,19 we adjusted for more than 1 year of experience as a potential confounder.…”

“…Finally, we employed a two-piece exponential hazard with a change of hazard at 12 months (complication 3). Two-piece exponential models are a special case of the Cox PH model; our model assumes a constant baseline hazard from 0–12 months on the job, and a separate constant baseline hazard from 12 months onward 19 26. Therefore, the final model was an AG model with a piecewise exponential baseline hazard, including a frailty term at the employee level.…”

“…In previous reports, we have assessed the roles of overtime, job demand, sex, work organisation and time on job, in addition to the study of obesity referenced above (Souza et al , unpublished data, 2013) 4 16–18. Using an Andersen–Gill (AG) model with a piecewise exponential baseline hazard, and employee-level frailty term, we explore here the possibility that one or more prevalent chronic diseases or overall health status measured from health claims may explain some of the ‘within-individual’ risks previously modelled using a random frailty term 19 20 21. Unlike previous studies, the use of exclusive and complete administrative health, injury and covariate data on the workers, and extensive knowledge about the jobs and their intrinsic risks, allows us to circumvent many of the limitations of the studies summarised above.…”

Precarious employment and occupational accidents and injuries – a systematic review

“…Most study groups were general samples of the working population. Four of the studies used a population of blue-collar workers (14)(15)(16)(17) and two of the studies used populations based on health care workers (18,19). The exposure measurements came either from corporate administrations (eg, payrolls), national statistics registers, or were self-reported.…”

“…Only one study examined differences in distribution between fatal and non-fatal injuries, and no difference was found between temporary and permanent workers (26). Two studies of aluminum plant workers found that unionization was associated with a higher risk of occupational injuries (17,27). Figure 2 shows a summarization of the results and effect sizes of the 17 included studies, sorted by exposures researched.…”