# Maximal logit distance for propensity score matching

I use propensity score matching (PSM) with logit regression distance. What maximal propensity score (ps) distance in each pair of the treatment and control individuals can be allowed? What maximal difference between mean propensity scores of treatment and control groups can be tolerated?

• PLeadse define your terms! What is PSM? What is PS? Nov 29, 2015 at 16:59
• @kjetilbhalvorsen Propensity score matching and propensity score. Nov 29, 2015 at 17:03
• Ple<zse add that as an edit to the question, so it becomezs legible! comments may be deleted! Nov 29, 2015 at 17:05

This is a suggestion that you consider not using PSM as a matching algorithm.

Gary King has a very recent paper (July 2015) on Why Propensity Scores Should Not Be Used for Matching that is pretty compelling. His suggestion is that Mahalanobis distance functions be used instead. It's worth reading the paper for his development of PSMs as well as MD functions. Wrt your questions about cutpoints, King's suggestion, on p. 20, is that they "be arranged so they divide the space between the minimum and maximum values into equal sized bins," basing a final decision on an imbalance metric (details in a link to an earlier paper given in footnote on p. 19).

http://gking.harvard.edu/files/gking/files/psnot.pdf?m=1439838506

In this ICM webinar (Sept 2015), he explains all of this succinctly and cogently, graphically demonstrating the significant improvement in matching accuracy of MD vs PSM.

http://www.methods-colloquium.com/#!Gary-King-Why-Propensity-Scores-Should-Not-Be-Used-for-Matching/clv6/55f31b8b0cf20cc524a66366

Some answers may be found in the following papers:

Austin P.C. Optimal caliper widths for propensity-score matching when estimating differences in means and differences in proportions in observational studies. http://onlinelibrary.wiley.com/doi/10.1002/pst.433/full

Optimal Caliper Width for Propensity Score Matching of Three Treatment Groups: A Monte Carlo Study http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0081045