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Re: st: propensity score matching

From   Steven Samuels <>
Subject   Re: st: propensity score matching
Date   Fri, 9 May 2008 19:15:30 -0400

On May 8, 2008, at 3:51 PM, Charman, Susan wrote, in part.

1. I was under the impression that "standard" nearest neighbour matching is done without replacement (matches removed)
Morgan and Harding (2006, p. 34) suggest that, among the basic algorithms, with-replacement nearest neighbor matching is superior to without-replacement matching. So, the default choice in -psmatch2- is probably deliberate.

and yet psmatch2 creates the _weight for which I get very high values ( e.g n=45) . this suggests that a subject is used 45 times as a match ( there are other subjects available with the same propensity score but not used).

Perhaps this, from the -psmatch2- help, is relevant:

"Note that the sort order of your data could affect the results when using nearest-neighbor matching on a propensity score estimated with categorical (non-continuous) variables. Or more in general when there are untreated with identical propensity scores."

Stephen L. Morgan and David J. Harding Theory and Practice Matching Estimators of Causal Effects: Prospects and Pitfalls in Sociological Methods Research 2006; 35; 3-60

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