Date:         Thu, 18 Sep 2008 11:39:06 -0700
Reply-To:     jn_mao@yahoo.com
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         jn mao <jn_mao@YAHOO.COM>
Subject:      Re: Contrast question-different number of degree
Comments: To: Robin R High <rhigh@UNMC.EDU>
In-Reply-To:  <OFC93C711F.249DBB34-ON862574C8.005E9531-862574C8.00611896@unmc.edu>
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Thanks Robin,
 
Here is my mixed model code,
proc mixed ;
class group subj hr;
model y=group hr group*hr /s;
contrast "overall 1 VS 2" group 1 -1  ;
contrast "1 vs 2 by hours" group 1 -1 group*hr 1 0 0 0 -1 0 0 0,
                   group 1 -1 group*hr 0 1 0 0 0 -1 0 0,
                   group 1 -1 group*hr 0 0 1 0 0 0 -1 0,
                   group 1 -1 group*hr 0 0 0 1 0 0 0 -1;
random group /subject=subj type=vc;
run;
 
I wonder why my Den DF is 123, but 41, and my total subject is 44. Any corrections on my code?
 
Jane


--- On Thu, 9/18/08, Robin R High <rhigh@UNMC.EDU> wrote:

From: Robin R High <rhigh@UNMC.EDU>
Subject: Re: Contrast question
To: SAS-L@LISTSERV.UGA.EDU
Date: Thursday, September 18, 2008, 12:40 PM

Jane,

It would help to see your code for the MIXED model and example of what
time represents.  You can combine the four contrasts into one statement to
get the NumDF=4, but don't see why you particularly want the DenDF=41 when
an ANOVA table shows that  degrees of freedom for the error term (with
complete data for all 44 subjects) for the group*hr effect is.

Source                     DF

group                       1
subj(group)                42
hr                          3
group*hr                    3
hr*subj(group)            126 ***


PROC MIXED nobound;
CLASS group hr subj;
model y = group hr group*hr ;
contrast "1 vs 2" group 1 -1 group*hr 1 0 0 0 -1 0 0 0,
                  group 1 -1 group*hr 0 1 0 0 0 -1 0 0,
                  group 1 -1 group*hr 0 0 1 0 0 0 -1 0,
                  group 1 -1 group*hr 0 0 0 1 0 0 0 -1;
random subj(group);
run;


If necessary, you can change the degrees of freedom for the three fixed
effects with the ddf= option (rather than ddfm=contain is default for
random effects)

And depending on what time is, perhaps a REPEATED statement with ar(1) or
something that takes into account a time related correlation structure
would work better, that is, replace the RANDOM statement with:

REPEATED hr / subject=subj(group) type=AR(1) rcorr;


Robin High
UNMC





jn mao <jn_mao@YAHOO.COM>
Sent by: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
09/18/2008 11:42 AM
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Subject
Contrast question






Hello SAS-Ls,I'm trying to compare different of outcome1 between two
groups by each time pointusing mixed model and random method.I have 44
subjects in 2 groups and 4 time point meansurements per subject.My
contrast code is:contrast "overall group1 VS group2" group 1
-1;contrast
"1 vs 2 at 1st hour" group 1 -1 group*hr 1 0 0 0 -1 0 0 0;contrast
"1 vs 2
at 2nd hour" group 1 -1 group*hr 0 1 0 0 0 -1 0 0;contrast "1 vs 2 at
3rd
hour" group 1 -1 group*hr 0 0 1 0 0 0 -1 0;contrast "1 vs 2 at 4th
hour"
group 1 -1 group*hr 0 0 0 1 0 0 0 -1; However, this code gave the contrast
Num DF and Den DF based on total data (Num DF=1, Den DF=123)how could I
code to get NumDF=4, and Den DF=41, or the degree is based on total
subjects? Thanks much! Jane