Re: Nagaraja

Date: Fri, 11 Jun 1999 08:18:14 -0700 From: LSheiner <lewis@c255.ucsf.edu> Subject: Re: Nagaraja Pascal, Assuming the baseline model does not have inter-occasion variability as a separate component, it makes sense that introducing a time-varying intra-individual covariate will decrease apparent intra-individual variability (epsilon variance): some of this variability was really inter-occasion variabiliy, which is now partly explained by the occasion-specific covariate. That it reduces the additive, ratherthan the multiplicative part of the error variance has no particular meaning to me. BeforeI paid too much attention to it, I'd try fixing the additive part to its original value, and seeing if the multiplicative part can compensate so as to yield almost as good a fit. I also can't say I have any immediate intuition about why inter-individual variability should go up; my guess would have been that it would stay about the same, and you do say "slightly," so perhaps it, too, is not meaningful (again, you might try fixing it to the original value and see if the GOF appreciably worsens). Lewis. Lewis B Sheiner, MD Professor: Lab. Med., Biopharm. Sci., Med. Box 0626 voice: 415 476 1965 UCSF, SF, CA fax: 415 476 2796 94143-0626 email: lewis@c255.ucsf.edu