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st: Difference between using dummy variables and the '==1' command


From   lreine ycenna <lreine.ycenna@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Difference between using dummy variables and the '==1' command
Date   Wed, 24 Aug 2011 15:58:27 +0100

Hi all,

I compared the regression results based on two different comamnds,
namely those derived from dummy variables and the 'if female==1'
command and found that at times these results can be quite different,
though they can also be very similar in some cases. For example,
comparing results from two separate regressions - (1) 'regress A B
AxFemale BxFemale' (from which I then add up the coefficients for A
and AxFemale), and (2) 'regress A B if female==1'.  Normally, at least
I expect, the coeffcients for A and female should be very similar
under these two commands, but I am unable to explain why sometimes
these coeffcients (e.g. A and Female) are very different depending on
the method used (e.g. 0.6 v.s -0.02).  I am wondering whether this is
normal? Can anyone explain to me whether there is any significant
methodological difference between these two comamnds?

Thanks.
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