Nonparametric identification of causal effects with confounders subject to instrumental missingness

We consider causal inference from observational studies when confounders have missing values. When the confounders are missing not at random, causal effects are generally not identifiable. In this article, we propose a novel framework for nonparametric identification of causal effects with confounders missing not at random, but subject to instrumental missingness, that is, the missing data mechanism is independent of the outcome, given the treatment and possibly missing confounder values. We also give a nonparametric two-stage least squares estimator of the average causal effect based on series approximation, which overcomes an ill-posed inverse problem by restricting the estimation space to a compact subspace. The simulation studies show that our estimator can correct for confounding bias when confounders are subject to instrumental missingness.