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st: AW: -areg- question

From   "Martin Weiss" <[email protected]>
To   <[email protected]>
Subject   st: AW: -areg- question
Date   Thu, 17 Sep 2009 22:57:43 +0200


Are you implying that -areg- and -regress- results must differ? -areg-, as I
understand it, is meant to replicate -regress- output when there are too
many fixed effects for -regress- to handle. Example 1 in [R], p. 80, shows
how the two commands return the same output:

sysuse auto, clear
regress mpg weight gear_ratio b5.rep78
areg mpg weight gear_ratio, absorb(rep78)


-----Ursprüngliche Nachricht-----
Von: [email protected]
[mailto:[email protected]] Im Auftrag von Sue
Gesendet: Donnerstag, 17. September 2009 19:28
An: [email protected]
Betreff: st: -areg- question

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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