The error message is telling you that the BMI for one of the subjects changed during the course of the repeated observations. It says that in a random effects model, the variables in the random effects are assumed to be constant within each subject.    > Would greatly appreciate any insight into how to correct code/what I am doing wrong.   If you expect BMI to vary within subjects, you might need to change your model and incorporate BMI as a fixed effect. Or, I suppose you could use the average BMI for each subject during the study if the BMI is changes very little. ... View more