Hi, SAS experts, I have used the following codes to analyze my data:   proc mixed data=ABC covtest noclprint noinfo noitprint method=reml; title3 'TOEP time and time2'; class id ; model fam_rou=time_c time2_c/ solution notest ddfm=kr ; random intercept time_c time2_c/ subject=id ; repeated / type=TOEP subject=ID r; run;   In this analysis, the with-individual residuals covariance matrix was modeled as the toeplitz structure. However, the SE of the residual (sigma2) was 0, leading no z-value and p value.    Does anyone know the reason why I got SE=0 and have any suggestions for addressing this issue? I appreciate it. ... View more