```Date: Thu, 18 Sep 2008 15:27:05 -0500 Reply-To: Robin R High Sender: "SAS(r) Discussion" From: Robin R High Subject: Re: Contrast question-different number of degree Comments: To: jn_mao@yahoo.com In-Reply-To: <402718.59537.qm@web32708.mail.mud.yahoo.com> Content-Type: text/plain; charset="US-ASCII" Jane, It can be enlightening to also run this model with GLM since for a random effects model it is basically equivalent to MIXED, and GLM prints the expected mean squares of the contrasts: proc glm data=one ; class group subj hr; model y=group subj(group) hr group*hr /ss3; contrast "overall 1 VS 2" group 1 -1 / e=subj(group); 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 / e=subj(group); test h=group e=subj(group); random subj(group) ; run; quit; with expected mean squares: Contrast Contrast Expected Mean Square overall 1 VS 2 Var(Error) + 4 Var(subj(group)) + Q(group,group*hr) 1 vs 2 by hours Var(Error) + Var(subj(group)) + Q(group,group*hr) indicating both contrasts should be tested by the subj(group) random term (as the contrast option allows you to specify) thus the need for 42 degrees of freedom; In MIXED you can specify the actual number for the degrees of freedom as an option on the CONTRAST statement: proc mixed data=one; 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 / df=42; *random group /subject=subj type=vc; random subj(group) ; run; the df are now 42 for this test in both GLM and MIXED; however the resulting Fvalue isn't quite the same, and I'm not aware how one can mimic the e=subj(group) in this situation as with GLM. [NOTE: I typically prefer the second RANDOM statement above, as the layout of the first reminds me of random intercepts or coefficients model where a numerical variable would usually be entered, plus it allows you to enter method=type3 on the PROC statement (closer to GLM) which isn't allowed otherwise.] Robin High UNMC jn mao 09/18/2008 01:39 PM Please respond to jn_mao@yahoo.com To Robin R High cc SAS-L@LISTSERV.UGA.EDU Subject Re: Contrast question-different number of degree 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 wrote: From: Robin R High 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 Sent by: "SAS(r) Discussion" 09/18/2008 11:42 AM Please respond to jn_mao@yahoo.com To SAS-L@LISTSERV.UGA.EDU cc 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 ```

Back to: Top of message | Previous page | Main SAS-L page