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Re: st: Box-Tidwell Test


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Box-Tidwell Test
Date   Tue, 26 Apr 2011 13:48:34 +0100 (BST)

--- On Tue, 26/4/11, lisenger@students.mail.uni-mannheim.de wrote:
> But I have another questions regarding testing the linearity
> assumption.

Your explanatory variable is a dummy/indicator variable, so there
is no linearity assumption to test. Think of it this way: Say we
have age as an explanatory variable and we enter it linearly, than
we can think of that as having a group of 25 year-olds, 26 year-
olds, etc. So we compare the expected outcome of 25 year-olds with
26 year olds, 26 year olds and 27 year olds, etc. The linearity
assumption is the assumption that each of these 1 year comparisons
are equal. It is this constraint that can be tested. If we only
have two groups than there is only 1 comparison, so there is no
possibility to constrain multiple comparisons to be equal. If there
is no constraint, there is no assumption, and there is nothing to 
test.

Hope this helps,
Maarten

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

http://www.maartenbuis.nl
--------------------------

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