The Trouble with Coarsened Exact Matching

37 Pages Posted: 7 Oct 2020

See all articles by Bernard S. Black

Bernard S. Black

Northwestern University - Pritzker School of Law; Northwestern University - Kellogg School of Management; European Corporate Governance Institute (ECGI)

Parth Lalkiya

Northwestern University

Joshua Y. Lerner

Duke University - Department of Political Science

Date Written: October 6, 2020

Abstract

“Balancing” methods, using matching or reweighting to improve the balance between treated and control units, have become central methodological tools for causal inference in the social sciences using cross-sectional observational data. We address here one method which has attained substantial popularity in political science, Coarsened Exact Matching (CEM) (Iacus, King, and Porro 2012). We report evidence that CEM performs substantially worse than an array of other methods and explain why it does so. We replicate five recent papers that use CEM and compare CEM-based results to those from other balancing methods. CEM drops substantially more observations, does so in non-obvious ways, can severely misidentify average treatment effects, and is much less precise than other methods. Our advice: never use CEM as the sole balancing method, and there is little to be said for using it at all.

The Online Appendix for this paper is available at http://ssrn.com/abstract=3705007.

Suggested Citation

Black, Bernard S. and Lalkiya, Parth and Lerner, Joshua Y., The Trouble with Coarsened Exact Matching (October 6, 2020). Northwestern Law & Econ Research Paper Forthcoming, Available at SSRN: https://ssrn.com/abstract=3694749 or http://dx.doi.org/10.2139/ssrn.3694749

Bernard S. Black (Contact Author)

Northwestern University - Pritzker School of Law ( email )

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Northwestern University - Kellogg School of Management

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European Corporate Governance Institute (ECGI)

Brussels
Belgium

Parth Lalkiya

Northwestern University ( email )

2001 Sheridan Road
Evanston, IL 60208
United States

Joshua Y. Lerner

Duke University - Department of Political Science ( email )

140 Science Drive (Gross Hall), 2nd floor
Duke University Mailcode: 90204
Durham, NC 27708-0204
United States

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