While I am aware of the QRPEM limitation in the earlier versions of NLME as far as nonlinear and time-varying (occasion) covariates, I am attempting to code a proportional effect of a lag time in a model using the following code (excerpt from my project):

*covariate(FAST)*

*dosepoint(Ag, tlag=tlag)*

*TVLAG=**tvtlag*(1 + (1-FAST)*theta_lag)*

*stparm( tlag= TVLAG*exp(ntlag) )*

*fixef( theta_lag = c(0, 0.75, ) )*

*fixef( tvtlag = c(0,0.5,3) )*

*FATAL ERROR IN QRPEM/IMPEM!*

*Model not suitable for QRPEM analysis*

*Possibly non linear covariate model or*

*some other unimplemented feature*

However, there is nothing nonlinear, nor time dependent about this covariate in the data set as the data set is only from the first dose. Thanks in advance!