Dear List,
I want to use PROC MIXED to assess the effect of an intervention on an outcome variable.The model will have two independent predictors/factors: trial (2 levels) and time (3 levels). All subjects (n=8) completed both trials.
In order to determine the appropriate covariance type for the model I want to start by running PROC MIXED with an unstructured covariance. However, I receive the following error: An infinite likelihood is assumed in iteration 0 because of a nonpositive definite estimated R matrix for Subject 1.. There is no error regarding the data file (i.e., no subject has the a condition/time point represented twice). The model runs fine if I use compound symmetry. Below is the syntax that I have used:
proc mixed data=Workrate;
class Subject Trial Time;
model Watts=Trial Time Trial*Time;
repeated Time / subject=Subject type=un r rcorr;
run;
Any help regarding this problem will be greatly appreciated.
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