RE: Using covariates with positive and negative values

From:"Bachman, William" Subject:RE: [NMusers] Using covariates with positive and negative values Date: Mon, 5 May 2003 12:34:07 -0400 couple of observations: 1. do you really want to add an exponential term or multiply by the exponential term: CL=TVCL *EXP(ETA(1)) V=TVV* EXP(ETA(2)) 2. in the absence of a meaningful estimate of the covariate effect, I suggest using a small value to only minimally perturb the system from the base model, e.g your theta(4) and theta(5) are large and causing the parameters to go negative. Try something like 0.001 instead of 1 for their estimates and use the final estimates from your base model for the other parameter estimates. That way you start off no worse than the base model. 3. Once you get your model to at least begin to iterate, you can use NOABORT on the $EST record to let the minimization proceed in the presence of negative parameters. nmconsult@globomaxnm.com GloboMax LLC 7250 Parkway Drive, Suite 430 Hanover, MD 21076 Voice: (410) 782-2205 FAX: (410) 712-0737