RE: covariate selection question

From: Michael.J.Fossler@gsk.com Subject: RE: [NMusers] covariate selection question Date: Tue, 17 Jan 2006 08:20:24 -0500 What you describe happens frequently. Two (or more) covariates may not have much influence by themselves, but together they influence the fit to a significant extent. However, I would not obsess over the -2LL as a sole criterion by which to judge the fit. Ask yourself the following questions: Does the inclusion of the covariate a) decrease the standard error of the relevant parameter,b) improve the fit (as judged by plots), c) make biologic sense? I would urge you not to rely on the -2LL as the sole criterion - I have seen too many examples where inclusion of a covariate decreased the -2LL but had a negative impact on the fit. Mike ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Michael J. Fossler, Pharm. D., Ph. D., F.C.P. Director Clinical Pharmacokinetics, Modeling & Simulation GlaxoSmithKline (610) 270 - 4797 FAX: (610) 270-5598 Cell: (443) 350-1194 Michael_J_Fossler@gsk.com ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~