st: Losing Observations in Logit

[Isobel Williams <iwilliams24@hotmail.com>]

Dear All,

I am running a logistic regression, and then pairing them using propensity score matching. Within the dataset, I have divided the data into groups A, B, and C:

generate groupA=1 if zona==1 
replace groupA=1 if cla_tam==1
replace groupA=1 if h_selec==1
replace groupA=0 if zona==0
replace groupA=0 if cla_tam==2
replace groupA=0 if cla_tam==3
replace groupA=0 if h_selec==0
replace groupA=0 if h_selec==9 


generate groupB=1 if zona==1
replace groupB=1 if cla_tam==1
replace groupB=1 if h_selec==0
replace groupB=0 if zona==0
replace groupB=0 if cla_tam==2
replace groupB=0 if cla_tam==3
replace groupB=0 if h_selec==1
replace groupB=0 if h_selec==9

generate groupC=1 if zona==0
replace groupC=1 if cla_tam==1
replace groupC=1 if h_selec==0
replace groupC=0 if zona==1
replace groupC=0 if cla_tam==2
replace groupC=0 if cla_tam==3
replace groupC=0 if h_selec==1
replace groupC=0 if h_selec==9

when I run the tab command for all groups, Stata tells me that:

. tab groupA

     groupA |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |     45,316       58.47       58.47
          1 |     32,187       41.53      100.00
------------+-----------------------------------
      Total |     77,503      100.00

. tab groupB

     groupB |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |     63,277       81.64       81.64
          1 |     14,226       18.36      100.00
------------+-----------------------------------
      Total |     77,503      100.00


. tab groupC

     groupC |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |     56,780       73.26       73.26
          1 |     20,723       26.74      100.00
------------+-----------------------------------
      Total |     77,503      100.00


However, when I run a logistic regression on propensity score matching between groups A and B, Stata tells me that I only have 7099 observations. 

Furthermore, when I wrote the code for keep if, the programme says "63227 observations deleted".

Here is what I tried to do in estimating a logit propensity score and match (nearest neighbor) between groups A & B:

preserve
keep if groupA==1 & groupB==1 
logit treat floor fmiss wall hhinc2....
predict double ps1
psmatch2 treat, outcome (S06A20) pscore (ps1) caliper(0.2) common logit
restore

The objective is to estimate a logit/propensity score and match observations from group A with group B. Any help on this matter would be very much appreciated.

Many thanks,
Isobel Williams 		 	   		  
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