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RE: adding error term to covariate

From: LI,HONG <lh1526>
Date: Thu, 31 Jul 2008 18:43:14 -0400 (EDT)

Dear group,

Thanks for your suggestions and the reference paper.
But I still have question that what we do expect from
incorprating error term to covariate considering the issue of
model fitting.

I incorprated error term to covariate using method B2 in the paper
there is not any significant improvement in model fitting and
parameter estimate. Dose that mean in my case residual error in
covaraite is not really important? Or because residual error is
small (< 10%), it is not necessary to worry about.



On Wed Jul 30 15:03:01 EDT 2008, "Samtani, Mahesh [PRDUS]"

> Hello,
> I was bit by this problem once in the past. My quick fix was to
> put etas on Smax & baseline (both parameters were estimated) and
> then put a block between these etas. It has a few limitations a)
> A fixed effect is modeled as a random effect; b) post-hoc plots
> of individual Smax and baseline were needed to visualize how the
> baseline influenced Smax; c) This forces a linear correlation
> when in fact the correlation can be curvilinear.
> Despite these limitations, the quick fix worked very well and I
> could meet my tight deadline. Can others point out other pros and
> cons of the above methodology.
> Thanks in advance for your feedback...Mahesh
> -----Original Message-----
> From: owner-nmusers
> [mailto:owner-nmusers
> Sent: Tuesday, July 29, 2008 4:52 PM
> To: LI,HONG; nmusers
> Subject: RE: [NMusers] adding error term to covariate
> Dear Li,
> You are right in thinking that your baseline better not be
> treated as an
> ordinary covariate (where we pretend that the covariate values
> are
> measured without error). Unless you want to be very restrictive
> in how
> to use the model (e.g. change from baseline in a study with
> similar
> design and patients with same baseline distribution), it is best
> to
> estimate the baseline. This also allows you to investigate
> relations (on
> the individual level) between baseline and drug effect/disease
> progression, etc.
> I am sure you will find many old threads on this topic if you
> search the
> nmusers archive. Additionally, here is a recent article:
> Related Articles, LinksDansirikul C, Silber HE, Karlsson
> MO.Approaches
> to handling pharmacodynamic baseline responses.J Pharmacokinet
> Pharmacodyn. 2008 Jun;35(3):269-83. Epub 2008 Apr 30.
> Cheers
> Jakob
> -----Original Message-----
> From: owner-nmusers
> [mailto:owner-nmusers
> On Behalf Of LI,HONG
> Sent: 29 July 2008 16:06
> To: nmusers
> Subject: [NMusers] adding error term to covariate
> Dear group,
> I have a baseline as covariate in the model, and it is the same
> measurement as the modeled variable. To me, it is reasonable to
> believe that there is measurement error for this covariate. Is
> there a way to incorporate some kind of error term into this
> baseline covariate? Thanks a lot.
> Graduate student
> University of Florida
> School of Pharmacy
> Department of Pharmaceutics
> Office: P4-10
> Phone: (352)273-7865

Graduate student
University of Florida
School of Pharmacy
Department of Pharmaceutics
My office: P4-10
Phone: (352)273-7865
Received on Thu Jul 31 2008 - 18:43:14 EDT

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