Date:         Mon, 5 May 1997 16:07:21 +0200
Reply-To:     Hans-Peter Piepho <piepho@WIZ.UNI-KASSEL.DE>
Sender:       "SAS(r) Discussion" <SAS-L@UGA.CC.UGA.EDU>
From:         Hans-Peter Piepho <piepho@WIZ.UNI-KASSEL.DE>
Subject:      Re: GLM and interaction contrasts
Comments: To: Jason Jones <jasonj@RCF.USC.EDU>
Content-Type: text/plain; charset="us-ascii"

>Does anyone know of a good place to find out about constructing =
>interaction contrasts in PROC GLM?  I can construct contrasts across =
>single factors fine, but not with interactions.  I know it's possible, I =
>just don't understand how they're constructed.
>
>For instance, in a 3*3 design, one possible contrast across either =
>factor would be -1 1 0.  However -1 0 0 0 0 0 0 0 1 is not a valid =
>interaction contrast, though that's how many means there are.
 
The model is overparamaterized. Therefore, contrasts among interactions are
not estimable. If you want contrasts among the 9 individual means of the 3 x
3 design, use the cell means model.
 
model y=a*b;
contrast '-11 + 33' -1 0 0 0 0 0 0 0 1;
 
_______________________________________________________________________
Hans-Peter Piepho
Institut f. Nutzpflanzenkunde  WWW:   http://www.wiz.uni-kassel.de/fts/
Universitaet Kassel            Mail:  piepho@wiz.uni-kassel.de
Steinstrasse 19                Fax:   +49 5542 98 1230
37213 Witzenhausen, Germany    Phone: +49 5542 98 1248