Practice of causal inference with the propensity of being zero or one: assessing the effect of arbitrary cutoffs of propensity scores

- Journal title : Communications for Statistical Applications and Methods
- Volume 23, Issue 1, 2016, pp.1-20
- Publisher : The Korean Statistical Society
- DOI : 10.5351/CSAM.2016.23.1.001

Title & Authors

Practice of causal inference with the propensity of being zero or one: assessing the effect of arbitrary cutoffs of propensity scores

Kang, Joseph; Chan, Wendy; Kim, Mi-Ok; Steiner, Peter M.;

Kang, Joseph; Chan, Wendy; Kim, Mi-Ok; Steiner, Peter M.;

Abstract

Causal inference methodologies have been developed for the past decade to estimate the unconfounded effect of an exposure under several key assumptions. These assumptions include, but are not limited to, the stable unit treatment value assumption, the strong ignorability of treatment assignment assumption, and the assumption that propensity scores be bounded away from zero and one (the positivity assumption). Of these assumptions, the first two have received much attention in the literature. Yet the positivity assumption has been recently discussed in only a few papers. Propensity scores of zero or one are indicative of deterministic exposure so that causal effects cannot be defined for these subjects. Therefore, these subjects need to be removed because no comparable comparison groups can be found for such subjects. In this paper, using currently available causal inference methods, we evaluate the effect of arbitrary cutoffs in the distribution of propensity scores and the impact of those decisions on bias and efficiency. We propose a tree-based method that performs well in terms of bias reduction when the definition of positivity is based on a single confounder. This tree-based method can be easily implemented using the statistical software program, R. R code for the studies is available online.

Keywords

causal inference;propensity score;positivity assumption;classification and regression tree;jackknife resampling;inverse propensity weighting;random forest;

Language

English

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