Dear Simon, I have two treatment groups , elderly and young , and i want to know the significance of age effect on pharmacokinetics of the new chemical entity. Can you please give me an example calculation or guide me thorugh the process. Thanking you with best regards, Raghav,
Raghav, in the NLME examples guide there is an example of adding covariates.(chapter 10, I think? My first question to you is do you really want to look at this as a categorical choice of elderly/young which may be somewhat arbitrary; might it not be better to look a AGE as potentially a continuous covariate; 
then you will be able to review the plot “PopCovariate Plots” to see any potential covariate effects, e.g. below we might see a trend for clearance to reduce with age 
Simon
