Cem: Coarsened Exact Matching in Stata

TLDR

Other matching methods inherit many properties of coarsened exact matching when applied to data preprocessed by it. The article introduces a Stata implementation of coarsened exact matching to improve causal effect estimation by reducing covariate imbalance. The cem command implements coarsened exact matching in Stata by temporarily coarsening data, exactly matching on the coarsened data, and then analyzing the uncoarsened matched data. Coarsened exact matching is faster, easier to use, requires fewer assumptions, and offers superior statistical properties, including bounded model dependence, monotonic imbalance control, no need for common support restriction, congruence, robustness to measurement error, full balance of nonlinearities and interactions, and compatibility with multiply imputed data.

Abstract

In this article, we introduce a Stata implementation of coarsened exact matching, a new method for improving the estimation of causal effects by reducing imbalance in covariates between treated and control groups. Coarsened exact matching is faster, is easier to use and understand, requires fewer assumptions, is more easily automated, and possesses more attractive statistical properties for many applications than do existing matching methods. In coarsened exact matching, users temporarily coarsen their data, exact match on these coarsened data, and then run their analysis on the uncoarsened, matched data. Coarsened exact matching bounds the degree of model dependence and causal effect estimation error by ex ante user choice, is monotonic imbalance bounding (so that reducing the maximum imbalance on one variable has no effect on others), does not require a separate procedure to restrict data to common support, meets the congruence principle, is approximately invariant to measurement error, balances all nonlinearities and interactions in sample (i.e., not merely in expectation), and works with multiply imputed datasets. Other matching methods inherit many of the coarsened exact matching method's properties when applied to further match data preprocessed by coarsened exact matching. The cem command implements the coarsened exact matching algorithm in Stata.