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Why Propensity Scores Should Not Be Used for Matching

Published online by Cambridge University Press:  07 May 2019

Gary King Open the ORCID record for Gary King [Opens in a new window]  and
Richard Nielsen Open the ORCID record for Richard Nielsen [Opens in a new window]
Gary King*
Affiliation:
Institute for Quantitative Social Science, Harvard University, 1737 Cambridge Street, Cambridge, MA 02138, USA. Email: [email protected], URL: http://GaryKing.org
Richard Nielsen
Affiliation:
Department of Political Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Email: [email protected], URL: http://www.mit.edu/∼rnielsen
*
*Email: [email protected]
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Abstract

We show that propensity score matching (PSM), an enormously popular method of preprocessing data for causal inference, often accomplishes the opposite of its intended goal—thus increasing imbalance, inefficiency, model dependence, and bias. The weakness of PSM comes from its attempts to approximate a completely randomized experiment, rather than, as with other matching methods, a more efficient fully blocked randomized experiment. PSM is thus uniquely blind to the often large portion of imbalance that can be eliminated by approximating full blocking with other matching methods. Moreover, in data balanced enough to approximate complete randomization, either to begin with or after pruning some observations, PSM approximates random matching which, we show, increases imbalance even relative to the original data. Although these results suggest researchers replace PSM with one of the other available matching methods, propensity scores have other productive uses.

Keywords

matchingpropensity score matchingcoarsened exact matchingMahalanobis distance matchingmodel dependence
Type
Articles
Information
Political Analysis , Volume 27 , Issue 4 , October 2019 , pp. 435 - 454
Copyright
Copyright © The Author(s) 2019. Published by Cambridge University Press on behalf of the Society for Political Methodology. 

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Footnotes

Authors’ note: The current version of this paper, along with a Supplementary Appendix, can be found at j.mp/PScore. We thank Alberto Abadie, Alan Dafoe, Justin Grimmer, Jens Hainmueller, Chad Hazlett, Seth Hill, Stefano Iacus, Kosuke Imai, Simon Jackman, John Londregan, Adam Meirowitz, Giuseppe Porro, Molly Roberts, Jamie Robins, Bradley Spahn, Brandon Stewart, Liz Stuart, Chris Winship, and Yiqing Xu for helpful suggestions, and Connor Jerzak, Chris Lucas, Jason Sclar for superb research assistance. We also appreciate the insights from our collaborators on a previous related project, Carter Coberley, James E. Pope, and Aaron Wells. All data necessary to replicate the results in this article are available at Nielsen and King (2019).

Contributing Editor: Jeff Gill

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