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Re: st: Multilevel model (xtmixed) question

From   Maarten buis <>
Subject   Re: st: Multilevel model (xtmixed) question
Date   Tue, 8 Dec 2009 11:51:12 -0800 (PST)

--- On Tue, 8/12/09, Rahsaan Maxwell wrote:
> My problem is that some of the independent variables have
> non-linear relationships with the DV.  Normally, I understood
> that to address this problem one should include the IV plus the
> IV squared. However, my variables are all standardized to mean
> zero so when I square them that obviously turns them into
> garbage.

I am guessing that you are confusing the logarithm with squaring,
there is no problem with adding a squared variable to a model
whose original values (before squaring) were sometimes negative.
Note however that a parabola (the original variable + the variable
squared) is a very special kind of non-linear relationship that 
could very well be inappropriate in your case. An attractive 
alternative that is more flexible are splines (see: -help mkspline-)

Hope this helps,

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


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