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Re: st: Interactions in xreg


From   Morten Hesse <mh@crf.au.dk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Interactions in xreg
Date   Mon, 23 Aug 2010 13:06:21 +0200


Den 23-08-2010 12:33, Maarten buis skrev:
--- 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
Germany

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




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