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Re: AW: st: reporting results from a multiple imputed data set

From   Maarten buis <>
Subject   Re: AW: st: reporting results from a multiple imputed data set
Date   Wed, 20 Apr 2011 15:24:40 +0100 (BST)

--- On Wed, 20/4/11, Lamla, Bettina wrote:
> (1) i can only include  M-1 year dummies.
> If my command includes M year dummies anyway, stata will
> omit one dummy due to collinearity, i.e. the least
> significant one.
> (2) the least significant one varies across imputations.
> Hence I could just exclude let's say year 1 for all
> estimations and hence cannot report an estimate for this
> year
>  OR
> I let stata pick the year dummy to be excluded and as a
> result -after applying the rubin rule- have estimates for
> all M year dummies even though this would not be possible in
> the individual regressions

When you or Stata leaves out a dummy/indicator variable than
the remaining dummy/indicator variables represent comparisons
with that reference category. It matters whether you compare
a year with say 1991 or 2011: the coefficients will have a 
different interpretation. So if you try to combine combine
estimates that are based on different reference categories 
than that is a very bad thing: the results will be completely 
meaningless. So you must specify the reference category 
yourself, i.e. leave out one of the dummies.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen

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