Re: Using covariates with positive and negative values

From: Leonid Gibiansky Subject: Re: [NMusers] Using covariates with positive and negative values Date:Mon, 05 May 2003 12:35:25 -0400 Garry, The model that you used is incorrect, you should not use CL=TVCL +EXP(ETA(1)). Use CL=TVCL*EXP(ETA(1)) (or CL=TVCL+ETA(1) ). To account for weight covariate, use something like CL=TVCL*EXP( THETA(?)*(WT/WT0-1)+ETA(1)) where WT0 is the mean or median weight, and similarly for V. Also, with just one point per animal, you may have problems with you model since you will have three parameter per animal (two eta's and one sigma) to fit just one data point. If your sampling points are at fixed time relative to the dose, I would rather try simple concentration versus covariate model C(fixed time, the same for all animals) = A+B*sex + C*(WT/WT0-1). If not, you may try to remove random effect from either volume or clearance and see how this will affect the result (covariate can be added to the parameter even if there are no random effect on this parameter). Leonid