Indemnity Benefit Duration and Obesity

Motivation. There is mounting evidence of obesity contributing to the cost of workers compensation. Longitudinal studies by Duke University (Østbye, Dement, and Krause [5]) of its own employees—and by Johns Hopkins University (Pollack et al. [7]) of employees of a multi-site U.S. aluminum manufacturing company—point to substantially higher odds of injury for workers in the highest obesity category. Further, a 2011 Gallup survey (Witters and Agrawal [9]) of 109,875 full-time employees found that obese employees account for a disproportionately high number of missed workdays, thus causing a significant loss in economic output. Finally, an NCCI study (Laws and Schmid [4]) of workers compensation claims established that where claimants are assigned a comorbidity code indicating obesity, the medical costs of the claim are a multiple of what is observed otherwise. In the following study, using a methodology similar to the one employed by Laws and Schmid [4], but accounting for a possible immortal time bias, it is shown that the indemnity benefit duration of claimants with an obesity-related comorbidity indicator is a multiple of what is observed for comparable non-obese claimants.

Method. The study makes use of a matched-pairs research design, where obese claims are matched with comparable non-obese claims in the data set. Exact matching applies to all claim characteristics, except age at injury, where proximity matching is employed. The set of matched pairs is then analyzed using a semiparametric Bayesian Weibull proportional hazard model, the nonparametric component of which accounts for the possible nonlinear influence of age. Aside from age, an indicator variable signifying obesity is the only covariate in the model—this is because net of these two covariates (and duration, which serves as the dependent variable), the claims within each set of matched pairs are identical. The model is estimated by means of MCMC (Markov chain Monte Carlo simulation).

Results. The study shows that, based on Temporary Total and Permanent Total indemnity benefit payments, the duration of obese claimants is more than five times the duration of non-obese claimants, after controlling for primary ICD-9 code, injury year, US state, industry, gender, and age. When Permanent Partial benefits are counted toward indemnity benefit duration as well, this multiple climbs to more than six.

Availability. The model was implemented in R (cran.r-project .org/) using the sampling platform JAGS (Just Another Gibbs Sampler, mcmc-jags.sourceforge.net/). JAGS was linked to R by means of the R package rjags(cran.r-project.org/web/packages/rjags/index.html).

Keywords. Obesity, Duration, Proportional Hazard Model, Semiparametric Model, Workers Compensation