Walid,
In the "old" days a researcher would have computed the means for each
day and analyzed those :( ... but there are other options (including a
totally random effects approach, which I won't illustrate).
MIXED offers the doublemultivariate option by adding a second
'repeated' factor on the REPEATED statement, called 'trial' or 'time' or
some variable entered into the dataset that is numbered 1 2 3 (see first
few records below).
Add time to the CLASS and REPEATED statements with one of the three
DblMlvt covariance options:
proc mixed data=data;
class pen TRT Day animal time;
model response= TRT Day TRT*Day ;
random pen;
repeated Day time / subject = animal(TRT) type = un@ar(1) r rcorr;
lsmeans TRTcode Day TRTcode*Day/diff;
run;
un@ar(1) is relevant if the time spans are equal each day. If not, other
choices are type=ar@cs and type=un@un
You may have convergence issues (at least I have with numbers in the
1000's). If so, dividing the response by 100 or some constant has
helped.
Robin High
University of Oregon
Original Message
From: SAS(r) Discussion [mailto:SASL@LISTSERV.UGA.EDU] On Behalf Of
Walid Alali
Sent: Wednesday, March 26, 2008 12:16 PM
To: SASL@LISTSERV.UGA.EDU
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 time
151 E 0 5 1 6.75E+03 1
151 E 0 5 1 6.22E+03 2
151 E 0 5 1 4.98E+03 3
152 E 0 5 10 6.25E+03 1
152 E 0 5 10 8.92E+03 2
152 E 0 5 10 6.09E+03 3
153 E 0 5 2 7.38E+03 1
153 E 0 5 2 4.68E+03 2
153 E 0 5 2 6.94E+03 3
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
Postdoctoral Research Associate
Department of Veterinary Integrative Biosciences
College of Veterinary Medicine and Biomedical Sciences
Texas A&M University
4458 TAMU
College Station, TX, USA 778434458
