st: propensity score matching

["Charman, Susan" <scharman@rcseng.ac.uk>]

Dear Stata users

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.

Many thanks
Susan

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