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st: psmatch2 question


From   anna bargagliotti <abargag@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: psmatch2 question
Date   Tue, 24 Aug 2010 09:42:17 -0700 (PDT)

Dear  Statalist, 
I am a bit confused on how to read  the output for the psmatch2 command.  I run 
the following:

psmatch2   bootcamp2008 sex race classlevel_2008 engineering_2008  
retorgradstem_2009, outcome(ucumgpa_2009) cal(.01)

where  bootcamp2008 indicates whether a student participated in the treatment 
group.  Sex,  race, classlevel_2008, engineering_2008, and retorgradstem_2009 
are a  set of characteristics I want to use to base the propensity score match  
on.  The outcome variable I am interested in is ucumgpa_2009.
I get  the following output: 

Probit regression                                 Number of obs   =       1433
                                                  LR chi2(5)      =      45.89
                                                  Prob > chi2     =     0.0000
Log likelihood = -130.35166                       Pseudo R2       =     0.1497

------------------------------------------------------------------------------
bootcamp2008 |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
         sex |   .3597951   .2029316     1.77   0.076    -.0379436    .7575338
        race |  -.0306858   .1762033    -0.17   0.862     -.376038    .3146664
classle~2008 |  -.4380801   .0876618    -5.00   0.000     -.609894   -.2662662
enginee~2008 |   .3163638   .1764673     1.79   0.073    -.0295058    .6622334
retorgr~2009 |   .1415633   .2017273     0.70   0.483     -.253815    .5369416
       _cons |  -1.581437   .2645664    -5.98   0.000    -2.099978   -1.062897
------------------------------------------------------------------------------
There are observations with identical propensity score values.
The sort order of the data could affect your results.
Make sure that the sort order is random before calling psmatch2.
----------------------------------------------------------------------------------------


        Variable     Sample |    Treated     Controls   Difference         
S.E.   T-stat
----------------------------+-----------------------------------------------------------


    ucumgpa_2009  Unmatched | 2.70719343   2.80314307   -.09594964   
.122931608    -0.78
                        ATT | 2.70719343   2.69239307   .014800359   
.218141977     0.07
----------------------------+-----------------------------------------------------------


Note: S.E. for ATT does not take into account that the propensity score is 
estimated.

           | psmatch2:
 psmatch2: |   Common
 Treatment |  support
assignment | On suppor |     Total
-----------+-----------+----------
 Untreated |     1,401 |     1,401 
   Treated |        32 |        32 
-----------+-----------+----------
     Total |     1,433 |     1,433  


My question is related to the T-stats reported (ie -.78 and .07).  I am  unsure 
how to interpret these.  What are these exactly?  I was under the  impression 
that the T-stat for ATT (.07) is the statistic to use in  determining whether 
the treatment had an effect on ucumgpa_2009,  however, the output does not give 
a significance level.  To check, I  tried to recreate this T-stat by computing 
it by brute force but I could  not do so.  

I tried the following: 
1.  reduced the sample to only those students who were in the treatment  and 
their matches
2. created repeated samples for students who were matches more than one  time
3. Performed the ttest comparing the ucumgpa_2009 of treatment vs.  control 
(indicated by the bootcamp2008 variable)

The tvalue I get is no where near the T-stat given by psmatch2.   Have I 
misunderstood the output given by psmatch2?  How do I tell  whether the 
bootcamp2008 had a significant effect on ucumgpa_2009?
Thank   you for your help.
Anna


      

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