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


From   Richard Williams <richardwilliams.ndu@gmail.com>
To   statalist@hsphsun2.harvard.edu, statalist@hsphsun2.harvard.edu
Subject   Re: st: Difference between using dummy variables and the '==1' command
Date   Wed, 24 Aug 2011 21:41:24 -0500

At 09:58 AM 8/24/2011, lreine ycenna wrote:
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?

If I understand you correctly you are not setting this up right. The first regression should be

reg A B female AxFemale BxFemale

i.e. you are leaving out the main effect of female. So, try something like

sysuse auto
reg price weight foreign i.foreign#c.weight
reg price weight if foreign ==0
reg price weight if foreign ==1

You'll see that the first regression gives you the same results as the next two regressions, just parameterized differently.


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Richard Williams, Notre Dame Dept of Sociology
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