As noted in Tips and Strategies for Mixed Modeling with SAS/STAT® Procedures by Kiernan et al., this warning could be due to an overspecified model. Try omitting   random intercept / subject=person_id; This paper by Littell et al. describes the difference between an AR(1)+RE structure and an AR(1) structure.   The Kiernan et al. paper lists other potential solutions to this problem.   I'd also add the <repeated-effect> to the REPEATED statement; it may not be necessary for your data but it's a programming safeguard. See the documentation.   I hope this helps.   ... View more