Re: [Fwd: CLIN PHAR STAT: Mixed Vs Fixed]
Date: Tue, 08 Aug 2000 17:48:43 -0700
From: LSheiner <lewis@c255.ucsf.edu>
Subject: Re: [Fwd: CLIN PHAR STAT: Mixed Vs Fixed]
Nick Holford wrote:
>
> Stephen,
>
> Stephen Senn wrote:
> >
> > I am very ignorant on PK/PD but the analogy here would seem to
> > be not in the number of patients but in the number of
> > measurements per patient. In this context, there may be a bias in
> > variance estimation and associated inferential statistics (CI, P-
> > values (ugh!) etc) for sparse sampling. However, it depends on the
> > way you set the model up. If you impose a common residual
> > variance for each patient then the problem largely disappears.
>
> It seemed to me that Mats had suggested a solution to this problem which
> was to explicitly use a DIFFERENT residual variance for each patient.
> The usual NONMEM model assumes a common residual error for each patient.
> What do you think the consequences of using different residual error for
> each patient might be?
>
What Mats suggests is to use a random effect on the residual eror - this means that you think of the error variance as being different for each person, but just like individual random effects ("etas" to us), this does not add a degree of freedom for each indivdiual, but only one df in total, for the estimated variance (of the variances).
LBS.
--
_/ _/ _/_/ _/_/_/ _/_/_/ Lewis B Sheiner, MD (lewis@c255.ucsf.edu)
_/ _/ _/ _/_ _/_/ Professor: Lab. Med., Bioph. Sci., Med.
_/ _/ _/ _/ _/ Box 0626, UCSF, SF, CA, 94143-0626
_/_/ _/_/ _/_/_/ _/ 415-476-1965 (v), 415-476-2796 (fax)