Re: st: Interactions in xreg

[Morten Hesse <mh@crf.au.dk>]

Thanks for your help!
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?
Greetings
Morten

Den 23-08-2010 10:40, Maarten buis skrev:
> --- On Mon, 23/8/10, Morten Hesse<mh@crf.au.dk>  wrote:
> > I have data, where subjects are tested in 5 blocks.
> > Subjects vary on  variable "ant" (dummy coded). I want
> > to test of there is a curvi-linear effect of "ant" on
> > outcome igt.
> > blocksq=block^2 (to test curvi-linearity).
> This really depends on what is in your variable block.
> If I see a variable names block I will by default assume
> that it is categorical. In that case, computing the
> square of that variable makes no sense. It could be an
> ordinal variable. In that case computing the square
> still makes no sense. You can only compute the square
> if you believe the values that the variable block can
> take also gives you information on the distances between
> the categories. Stata obviously cannot see what you
> believe, so it will alow you to compute a square of
> any variable, regardelss of whether that makes sense
> or not.
>
> If block is categorical or ordinal you must enter it
> as a categorical variable, e.g.:
>
> xtreg igt i.ant#i.block age gender, mle
>
> assuming you have Stata 11.
>
> 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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