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st: -areg- question


From   Sue <likecats@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: -areg- question
Date   Thu, 17 Sep 2009 13:28:09 -0400

Dear Statalisters,

I have a question on the number of observations used in -areg-. I run
the following regressions:

    reg `var'_mort ``var'_controls' _Irel* _Ibirth* _Icountry* _Iethn*
if touse`var' [pw = weight],
     cluster(clus_rc) robust

    areg `var'_mort ``var'_controls' _Irel* _Ibirth* _Icountry*
_Iethn* if touse`var' [pw = weight],
     absorb(mother_rc) cluster(clus_rc) robust

The only difference between the two is that the second is a regression
with additional fixed effects using the variable "mother_rc". I'm
wondering why the number of observations is the same in both
regressions, even though there are values of mother_rc(the group id)
that have only one observation and should therefore be dropped and not
used in the regression. I'm wondering if Stata drops those
observations but still reports the total sample size, or if there's
anything wrong. I'd really appreciate your input.
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