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Hi Walid,
Looks like a doubly repeated measures design, with both day and replicate as repeated measures. The following (UNTESTED) code should get you started down the right track.
proc mixed data=data;
class pen TRT Day animal replicate;
model response= TRT Day TRT*Day ;
random pen;
repeated replicate Day / subject = animal(TRT) type = un@ar(1) r rcorr;
lsmeans TRT Day TRT*Day/diff;
run;
Note the Kronecker product specification in the type. This assumes that your Day variable are equally spaced (according to your post, once a week).
I hope this helps. I would guess that if you don't do something with replicate, you are going to run into infinite likelihood problems with the repeated statement (multiple obs with identical qualifiers). The big problem with this approach is computing resources--that is going to be one big matrix to have in memory. If it doesn't work, you're probably going to have to fiddle around some with replicate as a random effect somehow. Maybe some of the other PROC MIXED gurus will jump in with a more elegant solution.
Steve Denham
Associate Director, Biostatistics
MPI Research
----- Original Message ----
From: Walid Alali <walidalali@GMAIL.COM>
To: SAS-L@LISTSERV.UGA.EDU
Sent: Wednesday, March 26, 2008 3:16:13 PM
Subject: Repeated measure in Mixed procedure
Hello all,
I am having trouble finding the best way of including all sources of
variability in my mixed model.
My study design (which is a clinical trial) goes like this: I have 3 pens,
in each pen; 10 animals (5 exposed and 5 controls). The response (i.e.
the outcome) is measured over time (every week for 2 months). To make this
complicated, the response was measured in triplicate (i.e. 3 times) at
each time point.
Here is a glimpse of the data (just for the 1st pen):
Animal TRT DAY Pen Replicate response
151 E 0 5 1 6.75E+03
151 E 0 5 1 6.22E+03
151 E 0 5 1 4.98E+03
152 E 0 5 10 6.25E+03
152 E 0 5 10 8.92E+03
152 E 0 5 10 6.09E+03
153 E 0 5 2 7.38E+03
153 E 0 5 2 4.68E+03
153 E 0 5 2 6.94E+03
154 E 0 5 3 2.49E+03
154 E 0 5 3 2.31E+03
154 E 0 5 3 2.03E+03
155 E 0 5 4 1.32E+04
155 E 0 5 4 9.92E+03
155 E 0 5 4 9.49E+03
156 C 0 5 5 4.22E+03
156 C 0 5 5 4.21E+03
156 C 0 5 5 3.46E+03
157 C 0 5 6 3.70E+03
157 C 0 5 6 3.62E+03
157 C 0 5 6 2.85E+03
158 C 0 5 7 1.54E+03
158 C 0 5 7 1.38E+03
158 C 0 5 7 1.63E+03
159 C 0 5 8 5.16E+03
159 C 0 5 8 5.00E+03
159 C 0 5 8 4.66E+03
160 C 0 5 9 5.37E+03
160 C 0 5 9 5.30E+03
160 C 0 5 9 5.41E+03
I have the model set up like this, but not sure how to include 'replicate'
in it??
proc mixed data=data;
class pen TRT Day animal;
model response= TRT Day TRT*Day ;
random pen;
repeated Day / subject = animal(TRT) type = un r rcorr;
lsmeans TRTcode Day TRTcode*Day/diff;
run;
Any comments, suggestions are very wellcome.
Thanks in advance,
Walid.
Walid Alali DVM, MS, PhD
Epidemiology
Post-doctoral Research Associate
Department of Veterinary Integrative Biosciences
College of Veterinary Medicine and Biomedical Sciences
Texas A&M University
4458 TAMU
College Station, TX, USA 77843-4458
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