Hi - I am using PROC MIXED to estimate a longitudinal  model. Below is a sample code   PROC MIXED DATA=TEST3 COVTEST NOCLPRINT NOITPRINT METHOD=ML; CLASS SUBID RACE; MODEL=TMA PRAC CAGE  RACE|CAGE/SOLUTION DDFM=BETWITHIN  OUTP=P ; RANDOM INTERCEPT CAGE/SOLUTION TYPE=UN SUBJECT=SUBID; RUN;   TMA is a continous outcome variable, PRAC is a cumumlative continous exposure variable (cumulative exposure to a toxic drug) and CAGE is a grand mean centered age variable, RACE is a three categroical level race variable.  My main predictor of interest is PRAC. My question is in order to estimate the association between PRAC and TMA - over duration of study, I include PRAC|CAGE interaction in the model?   Thank you for your help   ... View more