st: psmatch2 question

[Alistair Windsor <alistair.windsor@gmail.com>]

Dear Statalisters,

I am using psmatch2 to generate a matching group to estimate the average 
effect of the treatment on the treated. The group produced by these 
matching techniques is a proxy for the effect of not treating the 
treated (a counterfactual). There are various options to psmatch2, 
choices for stratification, and alternative matching approaches that I 
wish to compare. My first step is look at the summary statistics of the 
propensity scores and covariates over the treatment and matched groups.

Here is my problem: After running psmatch2 with the ties option

psmatch2 treatvar covariate_list, outcome(outcome_var) common ties ate

I expect to get _weight that sums to the number of observations in the 
treatment group (actually to the number of observations in the treatment 
group after common support is imposed using common). In my case this is 
98. However when I type

sum outcome_var [aweight = _weight] if _treated==0

I get a sum of weights of 92.924 (I am also unsure of what psmatch2 is 
storing in _weight for treated observations - as far as I can see 
nothing is mentioned in the documentation). I also expected the mean 
here to give me the estimate for the effect of not treating the treated 
- it is close but not equal to that returned by psmatch2. I certainly 
would like to be able to duplicate the output so I can be sure I know 
what is being done by the script.

I am sure that I am missing something. Can anyone enlighten me?

Alistair

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