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Re: st: Prediction at the 'mean' of a categorical- xi3?


From   graupel75@gmail.com
To   Statalist <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Prediction at the 'mean' of a categorical- xi3?
Date   Thu, 14 Mar 2013 11:59:08 +0800

JVerkuilen writes:

>Nothing in the math says you can't center dummy variables; I do it all
>the time using the usual Stata commands to make the intercept a
>meaningful number. Effects coding in some sense does this in the
>classic ANOVA setup but won't in a general regression due to the lack
>of orthogonality.

I've used both -xi3- center option and generated the mean-centered var
by hand, and run the regression with this new var, but the log lik and
AIC/BIC are quite different indicating that more than just the ref
category has changed...
so I must be confused.

Let's say I have a categorical (levels 1 to 6), do I then estimate
with the mean-centered value as a continuous var, and predict at 3.5?
This doesn't make so much sense as it would assume the steps between
levels are equivalent; I've looked in both Agressi and Long's Stata
book without success-

thanks!
Andrew

______________________________________________
Epidemiologist
Centre for Infectious Disease Epidemiology Research (CIDER)
Saw Swee Hock School of Public Health
National University of Singapore
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