11-24-2014 08:11 PM
I have two questions about the covariance structure in proc mixed.
11-25-2014 09:16 AM
For 1, it is actually fairly common for the covariance to be negative. I get this result a lot for some data structures and models. You are not correct that "the covariance is always positive". If subject is a block, then a negative covariance means that two randomly chosen observations within the same block are negatively correlated (or that there is more variation within blocks than between blocks). And for repeated measures, it is definitely possible for observations within a subject to have a negative correlation.
Can't answer your second question because the link is too general. Not clear what part you are asking about. But B is generic for main effect B (random effect) and AB is the interaction of A (presumed fixed) and B.
I highly recommend that you get a copy of SAS for Mixed Models, second edition (2006; SAS, Inc.), by Littell et al.