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Re: st: propensity score matching


From   David Kantor <kantor.d@att.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: propensity score matching
Date   Thu, 08 May 2008 16:02:12 -0400

At 03:51 PM 5/8/2008, Susan wrote:
I would welcome some advice on the use of psmatch2 - I seem to have become confused somewhere along the line.
I have estimated a propensity score using logistic regression. I now want to match on this and then use conditional logistic regression to assess the effect of treatment on outcome. I have approximately 11000 subjects on treatment A and 2300 on treatment B. I was hoping to create a matched data set with A:B in the ratio n:1 where 1<n<4. I had hoped that psmatch2 would do this with nearest neighbour matching without replacement. However I seem to be having some problems or am misunderstanding the help.

1. I was under the impression that "standard" nearest neighbour matching is done without replacement (matches removed) 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).
2. I would like to do 4:1 matching but cannot seem to find a way of identifying the id's of all the matches - is this possible. If not can I get the outcomes for all the matched pairs so that I can compute OR's.
[...]
I can't address all the issues, but may I suggest mahapick or mahascores in my mahapick package. This will give you as many nearest neighbors as you desire. BUT it does matching with replacement. You can, however, take the results and do you own juggling to obtain a matching-without-replacement result.

HTH
--David

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