RE: Decrease in OFV with a fixed effect

From:"Piotrovskij, Vladimir [PRDBE]" VPIOTROV@PRDBE.jnj.com Subject: RE: [NMusers] Decrease in OFV with a fixed effect Date: Tue, August 24, 2004 6:16 am Pete, I would recommend to keep all BIV-related random effects in the model until the full covariate model is finalized. Usually, due to practical reasons the base model (w/o covariate effects) implements a diagonal random effect matrix. It is not uncommon the data do not support estimation of BIV in all parameters, however, before you build the covariate model you cannot say, which component of the matrix is not estimable. If you drop one or more ETAs based on the fact that the corresponding OMEGAs are negligible in the base model you are at risk of selecting a wrong matrix for the final model. Best regards, Vladimir ----------------------------------------------------------------- Vladimir Piotrovsky, Ph.D. Research Fellow, Advanced PK-PD Modeling & Simulation Global Clinical Pharmacokinetics and Clinical Pharmacology (ext. 5463/151) Johnson & Johnson Pharmaceutical Research & Development Turnhoutseweg 30 B-2340 Beerse Belgium