NONMEM Users Network Archive

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RE: adding error term to covariate

From: Samtani, Mahesh [PRDUS] <MSamtani>
Date: Wed, 30 Jul 2008 15:03:01 -0400

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
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.

LI,HONG
Graduate student
University of Florida
School of Pharmacy
Department of Pharmaceutics
Office: P4-10
Phone: (352)273-7865

Received on Wed Jul 30 2008 - 15:03:01 EDT

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