Date:         Fri, 7 Apr 2000 23:35:26 GMT
Reply-To:     Dale McLerran <dmclerra@FHCRC.ORG>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Dale McLerran <dmclerra@FHCRC.ORG>
Organization: Fred Hutchinson Cancer Research Center
Subject:      Re: GLM: Simple effects for within subjs factors

[posted and mailed]

jweedon@earthlink.net (Jay Weedon) wrote in
<aCnuONUiX9wxjorQXkaSEvWnRzCZ@4ax.com>:

>Hi folks,
>
>Is there any easy way to get SAS to do post-hoc tests on
>within-subjects effects, using PROC GLM or otherwise? All the post-hoc
>options in GLM seem to focus only on between-subjects effects.
>
>Example: A mixed model ANOVA has 2 groups (A1, A2)  and 3 repeats
>(B1,B2,B3) per subject. I want to do LSD pairwise comparisons among
>B1,B2,B3 pooled across groups, and simple effects of B at A1, B at A2.
>
>TIA,
>Jay Weedon.
>

Jay,

How about using PROC MIXED as follows:

proc mixed;
  class A B SUBJECT;
  model resp = A|B / s;
  repeated B / subject=SUBJECT type=UN;
  lsmeans B / diff;
  lsmeans A*B / slice=A diff;

I specified an unstructured covariance matrix for the repeated measures.
You may choose a different covariance structure, but unstructured
should be the same as doing a repeated measures analysis using GLM.

The structure of the data must be different for a repeated measures
analysis using PROC MIXED than it is for PROC GLM.  You need one
record per time period by subject.  That is, you would have 3*Ns
observations, where Ns is the number of subjects in your dataset.
The first record for subject 1 has response at period B1 and variable
B=1, the second record for subject 1 has response at period B2 and
variable B=2, etc.

In this framework, there is no difference in how we analyze A and B,
except that we have specified that B is the variable which identifies
the order of repetitions.  I believe that this is easier to work with
than in the GLM framework.

Dale

--------------------------------------
Dale McLerran
Fred Hutchinson Cancer Research Center
Seattle, WA 98109
mailto:dmclerra@fhcrc.org
ph:  (206) 667-2926
fax: (206) 667-5977
--------------------------------------