Hi,   I have some questions about the difference between repeated measurements and nested models. For repeated measurements, we have different times nested within one id. For usual examples of nested models, we have different subjects nested within one school or states. So do they have any difference?   For example, if the id is nested within the city, and the city is nested within states. I wrote down the following codes. Are they equivalent? (It seems that they are not equivalent because SAS just stopped because of too many iterations for the first one) I don’t know why they are different.   proc mixed data=hello1217;       class unitid control State city;       model rate=control/solution;       random unitid(city) city(state) State; run;   proc mixed data=hello1217;       class unitid control State city;       model rate=control/solution;       random unitid(city) city /subject=state; run;   Also, if we have repeated measurements in time, for the following code, are they equivalent?   proc mixed data=test;       class time;       model y=x/solution;       random time /subject=id; run;   proc mixed data=test;       class time;       model y=x/solution;       random time(id) id; run;   Thank you!   Wenru ... View more