RE: adding error term to covariate
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
Quoted reply history
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of Ribbing, Jakob
Sent: Tuesday, July 29, 2008 4:52 PM
To: LI,HONG; [email protected]
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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of LI,HONG
Sent: 29 July 2008 16:06
To: [email protected]
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