How do i get a block diagonal covariance matrix, V, in SAS Proc Mixed or Proc glimmix?

[Lyson]

Hi friends,

I am trying to estimate a linear mixed model for treatments arranged in CRD using SAS using proc mixed or proc glimmix. I have 3 categorical factors A, B and C where factor C is assumed random, and A,B are fixed. My data set looks like this:

The  syntaxes that I am exploring are:

a) PROC GLIMMIX

proc glimmix data=FFRgroup outdesign=XZ;;
class factorA factorB factorC  groupA groupB groupC rep;
model yield= groupA | groupB / ddfm=satterthwaite;
random factorC factorA*factorC  factorB*factorC factorA*factorB*factorC /  V;
lsmeans groupA | groupB   / pdiff cl alpha=0.05 ;
Title "FFR Model";
run;

whch displays the following V matrix,

b) PROC  MIXED:

proc mixed data=FFRgroup ;
class factorA factorB factorC  groupA groupB groupC rep;
model yield= groupA | groupB / ddfm=satterthwaite;
random factorC factorA*factorC  factorB*factorC factorA*factorB*factorC /  V;
lsmeans groupA | groupB   / pdiff cl alpha=0.05 ;
Title "FFR Model";
run;

displays the following V matrix,

What could be the problem with these two approaches not producing the covariance structure that aligns with the model in question (block diagonal)? 

Thank you in advance for your assistance folks.

[jiltao]

If you rewrite your RANDOM statement below --

random factorC factorA*factorC  factorB*factorC factorA*factorB*factorC

to the one that makes the model to be processed by subjects and is more numerically efficient --

random int factorA  factorB factorA*factorB / subject=factorC v;

in both procedures, then you would probably see the V option displaying the first block (for the first level of FACTORC) of the variance covariance matrix. Do the two procedures give you the same output for the V matrix?

Also the values in the output you show from PROC MIXED look unusually large to me. What do you see in the Log from PROC MIXED? It might be a good idea to examine the logs from both procedures first.

Thanks,

Jill

[Mike_N]

When you use the V option of the random statement in either Proc Glimmix or Proc Mixed, the default behavior is to print out the first (single) block from the V matrix, not the whole block diagonal matrix. However, both procedures are still using block diagonal V. Note that blocks are usually defined by using the subject= option within the random statement. See here SAS Help Center: RANDOM Statement and here SAS Help Center: RANDOM Statement for more details about how to use the subject= statement to specify the blocks of V.