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Comparative Study
. 2011 Sep;22(5):718-23.
doi: 10.1097/EDE.0b013e31822549e8.

A comparison of methods to estimate the hazard ratio under conditions of time-varying confounding and nonpositivity

Ashley I Naimi  1 , Stephen R Cole, Daniel J Westreich, David B Richardson
Affiliations
Comparative Study

A comparison of methods to estimate the hazard ratio under conditions of time-varying confounding and nonpositivity

Ashley I Naimi et al. Epidemiology. 2011 Sep.

Abstract

In occupational epidemiologic studies, the healthy worker survivor effect refers to a process that leads to bias in the estimates of an association between cumulative exposure and a health outcome. In these settings, work status acts both as an intermediate and confounding variable and may violate the positivity assumption (the presence of exposed and unexposed observations in all strata of the confounder). Using Monte Carlo simulation, we assessed the degree to which crude, work-status adjusted, and weighted (marginal structural) Cox proportional hazards models are biased in the presence of time-varying confounding and nonpositivity. We simulated the data representing time-varying occupational exposure, work status, and mortality. Bias, coverage, and root mean squared error (MSE) were calculated relative to the true marginal exposure effect in a range of scenarios. For a base-case scenario, using crude, adjusted, and weighted Cox models, respectively, the hazard ratio was biased downward 19%, 9%, and 6%; 95% confidence interval coverage was 48%, 85%, and 91%; and root MSE was 0.20, 0.13, and 0.11. Although marginal structural models were less biased in most scenarios studied, neither standard nor marginal structural Cox proportional hazards models fully resolve the bias encountered under conditions of time-varying confounding and nonpositivity.

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Figures

FIGURE
FIGURE
Causal directed acyclic graph representing the healthy worker survivor effect, with time-varying exposure [X(0) and X(1)], work status [W(1)], an unmeasured confounder (U), and a time to event variable (T). Nonpositivity arises due to the zero probability of exposure at follow-up [X(1)] for those who have left work (W(1) = 0).
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Comment in

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