Re:

From: Alan Xiao <Alan.Xiao@cognigencorp.com> Subject: Re: Date: Thu, 16 Aug 2001 15:07:18 -0400 Hello, Dear Dr. Beal, Thanks for your comments. About the saddle point, how could it be formed and how to diagnose? Can (linear) addition of some extra covariates into the model change the surface around a point from a cone shape or whatever into a saddle shape surface? Or is this just from the structure of the model and not related to covariates at all (as you mentioned "fundamental" problems in the last email)? If yes, then how to explain that it's no problem for the structural model and all models with up to 23 covariates to go through $COV step by default? - You won't suggest that we can still get the right R on a saddle shape surface, right? On the other hand, if a saddle shape surface is so big that it covers the whole range of you data (wild?), what could you do? Thanks, Alan. -- ***** Alan Xiao, Ph.D *************** ***** PK/PD Scientist *************** ***** Cognigen Corporation ********** ***** Tel: 716-633-3463 ext 265 ******