Subpopulation

Dear NMusers: I am trying to fit a dataset with 13 dose levels. The highest dose is about 10 times of the lowest dose. Each patient receive one dose and were sampled intensively up to 7 days. The results of individual PK analysis shown linear kinetics for some of the patients and nonlinear kinetics for the other patients. I have tried to fit all of them together. But my advisor wants me to fit linear patients and nonlinear patients separately to get a better look of fitting. Additionally, all the nonlinear patients are from higher dose levels. But not all the patients in higher dose levels shown nonlinear kinetics. So my question is which way is more appropriate in this case? Should I fit them all together or separately? Could these two types of patients be considered as subpopulations? Any comment or suggestion will be highly appreciated. Best regards, Huali