We are trying to model response vs. concentration, with concentration as the independent variable. Using covariate(C) in the user-written model. The population model works fine and gives expected estimates for thetas. However when we go to run a Pred Check, the IVAR column is all zeros. Is the solution to reassign the IVAR column to concentration? How do we do that? mod1(){ covariate(C) drug = C/1000 E = Int + Slp*(drug) stparm(Int = tvlInt + nInt) stparm(Slp = tvlSlp) #* exp(nSlp)) fixef( tvlInt = c(, 1.285, ) tvlSlp = c(, -1.21, ) ) ranef( diag(nInt) = c(30.88) ) error(EEps = 8.61) observe(EObs = E + EEps) }