You can explore the Kronecker product structures for TYPE= on the REPEATED statement. That is a handy construct for two-dimension repeated measures. You can set up an unstructured covariance between the eyes of a patient and CS, AR(1), or (on occasion) UN for observations within an eye. The model you have now is used quite often as well. You could try adding covariance within an eye as an extra random effect, with random int eye / subject=ptid; That RANDOM statement correlates observations from the same patient and further correlates observations from the same eye within a patient. You could fit AR(1) as a structure on the REPEATED statement you have now with this RANDOM statement.  ... View more