Need Help with Proc MIXED, nested random factor removal

igforek — Wed, 06 Jul 2016 15:22:12 GMT

Hello,
I run an analysis by using PROC MIXED as implemented in the EG 7.1 software. I have the code below:

PROC MIXED DATA = WORK.SORTTempTableSorted covtest
PLOTS(ONLY)=ALL
METHOD=REML;
CLASS IsolStr IsolRepeatStr IsolVialStr InfStaStr;
MODEL TotFec=InfStaStr;
RANDOM IsolStr IsolRepeatStr(IsolStr) IsolVialStr(IsolRepeatStr IsolStr ) / TYPE=VC;
RUN; QUIT;

As can be seen in the code, I have a fixed factor, "InfStaStr," and several nested random factors. The random factors "IsolStr" and "IsolVialStr(IsolRepeatStr IsolStr)" had a varainace zero when the full model was run, but the nested term "IsolRepearStr(IsolStr)" did not a have a variance of zero (see attached file), and I get the "NOTE: Estimated G matrix is not positive definite."

I read in the SAS manual that in a case like this, random factors with variance zero can be romoved from the model. If I follow that advise, I would be left with a model with a single nested factor, "IsolRepearStr(IsolStr)," where the factor it is nested whitin, "IsolStr," is not present.

So, my question is: is it statistically sound to remove "IsolStr" in this case? Thank you for any help on this matter.
Igforek

Re: Need Help with Proc MIXED, nested random factor removal

lvm — Wed, 06 Jul 2016 18:10:43 GMT

For nested variance component (VC) effects, you can leave the 0-variance terms in the model. The results will be the same with or without them. Also, there is nothing wrong with leaving in a single random effects term as you described.

Re: Need Help with Proc MIXED, nested random factor removal

igforek — Thu, 07 Jul 2016 07:39:42 GMT

OK, it seems reasonable. I checked and there is not much change in the output when removing the random nested factors with value zero from the model. Thank you for your answer.

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Need Help with Proc MIXED, nested random factor removal

Re: Need Help with Proc MIXED, nested random factor removal

Re: Need Help with Proc MIXED, nested random factor removal