RE: Covariate effect

From: "Serge Guzy" Subject:RE: [NMusers] Covariate effect Date: Fri, 7 Mar 2003 13:58:23 -0800 Without entering into mathematical details, my impression is that an influential covariate should lead to a better objective function (ignoring the degree of freedom effect) and at the same time decrease the ETA. The use of a covariate separates to some extent the whole population into subset of populations, each of them characterized by different population means. Suppose the 1 compt model, bolus injection and V lognormal V=Vfixed*exp(Vrandom): Suppose Vfixed=exp(a+b*weight(i)) and the covariate weight is really significant(i for individual i). If I am coming back to each individual and plug their corresponding weight into the Vfixed term, I see that Vfixed will be different for every individual. If the covariate is influential, it will decrease the residual part for every individual(V random).Consequently, the eta will decrease. Serge Guzy President POP-PHARM(510 453 7443) Head of Pharmacometrics and Preclinical Statistics Xoma: tel(510) 2047476