Built-in selection or confounder bias? Dynamic Landmarking in matched propensity score analyses

Abstract

Background: Propensity score matching has become a popular method for estimating causal treatment effects in non-randomized studies. However, for time-to-event outcomes, the estimation of hazard ratios based on propensity scores can be challenging if omitted or unobserved covariates are present. Not accounting for such covariates could lead to treatment estimates, differing from the estimate of interest. However, researchers often do not know whether (and, if so, which) covariates will cause this divergence. Methods: To address this issue, we extended a previously described method, Dynamic Landmarking, which was originally developed for randomized trials. The method is based on successively deletion of sorted observations and gradually fitting univariable Cox models. In addition, the balance of observed, but omitted covariates can be measured by the sum of squared z-differences. Results: By simulation we show, that Dynamic Landmarking provides a good visual tool for detecting and distinguishing treatment effect estimates underlying built-in selection or confounding bias. We illustrate the approach with a data set from cardiac surgery and provide some recommendations on how to use and interpret Dynamic Landmarking in propensity score matched studies. Conclusion: Dynamic Landmarking is a useful post-hoc diagnosis tool for visualizing whether an estimated hazard ratio could be distorted by confounding or built-in selection bias.