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Re: st: Interactions in xreg

From   Maarten buis <>
Subject   Re: st: Interactions in xreg
Date   Mon, 23 Aug 2010 10:33:11 +0000 (GMT)

--- On Mon, 23/8/10, Morten Hesse wrote:
> I did that too, and it works. "Block" is of course an
> ordinal variable (1-5). Yet, I want to test whether 
> the group with characteristic "ant" has a different
> pattern of behaviour over "blocks", and I have reasons 
> to believe that that pattern is curvilinear, compared
> with subjects who do net have characteristic "ant"
> (scores increases towards the middle, of the test,
> decreases towards the end). I can of course test
> whether the parameter for subjects during the 
> different blocks are significant (e.g., start versus
> middle, and middle  versus start). And indeed, they
> are. But I have never seen that used as  a test of
> curvilinearity, and I feel that it is a bit "informal".
> Am I wrong?

Yes, unfortunately you are wrong. Such quadratic terms 
have no meaning in an ordinal variable. If you do so
you are basically assuming that the distances between
your 5 categories are equal.

-- Maarten

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


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