st: Propensity Score Matching Between 3 Groups

[ir w <iwilliams24@hotmail.com>]

Hi,
 
I am trying to estimate a propensity score and then find the 
average treatment effect on the treated. However, my sample has 3 
groups:
 
Group A: Resides in intervention area, receives treatment
Group B: Resides in intervention area, doesn't receive treatment
Group C: Resides in non-intervention area, doesn't receive treatment
 
Thisis effectively like having 1 intervention group and 2 control groups: I
 want to calculate a propensity score for treatment between Groups A and
 B. I then want to apply this propensity score to Groups A and C to find
 the average treatment effect on the treated. 
 
I have seen this done in several economic papers, but never explained thoroughly. Is it 
possible to do this in Stata, and if so, how?
 
If it is relevant, I am using a logit model and -psmatch2- (the user-written SSC programme) to estimate the propensity score.
 
Thanks!                                    
Isobel Williams 		 	   		  
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