st: The sample reduces when running logistic regressions

[liliana <lilly-90@hotmail.it>]

Hello,

I am running a logistic regression on different samples using multiple
imputations:

mi estimate, or post: logit Y X1 X2, vce(cluster PSU)

I have run the regression for the first samples and Stata gave me back
results where all the observations are included, except for one sample for
which I have got 1332 observations but 1292 observations were taken by Stata
for the regression. In addition, the categorical variable education
(i.education which takes values 0, 1, 2, and 3) presents a problem:
0.education is the baseline, for 1.education and 2.education Stata gives me
back the relative coefficient, whereas 3.education is omitted although there
are 168 observations. 

Which could be the causes of these two problems? 

Thanks in advance. 
Lil



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