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Re: st: Factor variable notation vs. hand made dummy vars


From   Richard Williams <richardwilliams.ndu@gmail.com>
To   statalist@hsphsun2.harvard.edu, "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Factor variable notation vs. hand made dummy vars
Date   Mon, 06 Feb 2012 10:43:19 -0500

At 10:25 AM 2/6/2012, Ulrich Kohler wrote:
Hi all,

I cannot replicate the model

. sysuse auto, clear
. tab rep78, gen(d)
. logit for mpg d2-d5

with factor variable notation. I tried

. logit for mpg ib1.rep78

but results differ. Can anybody explain why?

(Note as an aside that

. logit for mpg d1-d5

reproduces the factor variables solution, but normally I would not
specify the model this way)

Two additional cases get dropped using factor variable notation. I'm guessing that with the first approach, Stata is having trouble distinguishing perfect multicollinearity from extreme multicollionearity. rep78 has some very small Ns for some of its categories, which is one of the reasons I don't like to use it in any examples I do.


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Richard Williams, Notre Dame Dept of Sociology
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EMAIL:  Richard.A.Williams.5@ND.Edu
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