All the SAS output seems normal, and there is no warning or error in log, the estimate of covariance parameter can be obtained when the type is compound symmetry, but the standard error  is not estimable, I want to know why and is it a critical issue?  For some confidential reason, i can't show the data, but i can give a brife introduction to the data: the data is create for efficacy analyses for 2 drugs (0.01%  0.02%) vs placebo (represented by variable trt01pn), there are several repeated assessments (represented by variable avisitn1) for each subject, and at each assessment time point, 2 results will be obtained from 2 eyes(left or right, represented by variable oelat) respectively,  the response result to drug is success or failure(represented by variable response1), other variables used in model are covariates (like randage AMETROPI).   And here is the SAS code i use:    ods output estimates=out_est.;    proc glimmix data=data empirical method=quad;     class subjid trt01pn(ref='3') avisitn1(ref='6') oelat randage AMETROPI;    model response1= trt01pn avisitn1 oelat randage AMETROPI trt01pn*avisitn1 / dist=binary  link=logit solution or;    nloptions  maxiter=200;     random intercept / subject=subjid type=vc;    random intercept / subject=oelat(subjid) type=cs;    estimate '0.01% vs placebo at M12 ' trt01pn 1 0 -1 trt01pn*avisitn1 1 0 0 0 -1 0/ exp cl;     estimate '0.02% vs placebo at M12' trt01pn 0 1 -1 trt01pn*avisitn1 0 0 1 0 -1 0/ exp cl;    estimate '0.02% vs 0.01% at M12' trt01pn -1 1 0 trt01pn*avisitn1 -1 0 1 0 0 0/ exp cl;    run; Thank you very much for your help!   @Rick_SAS   @SteveDenham  ... View more