NONMEM Users Network Archive

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Re: OMEGA selection

From: Nick Holford <n.holford>
Date: Wed, 15 Apr 2009 18:17:57 +0200

Ethan,

Do not pay any attention to whether or not the $COV step runs or even if
the run is 'SUCCESSFUL' to conclude anything about your model. Your
opinion is not supported experimentally e.g. see
http://www.mail-archive.com/nmusers
discussion and references.

NONMEM has no idea if the parameters make sense or not and will happily
converge with models that are overparameterised. You cannot rely on a
failed $COV step or a MINIMIZATION TERMINATED message to conclude the
model is not a good one. You need to use your brains (NONMEM does not
have a brain) and your common sense to decide if your model makes sense
or is perhaps overparameterised.

Nick

Ethan Wu wrote:
>
> Dear all,
>
> I am fitting a PD response, and the equation goes like this:
>
> total response = baseline+f(placebo response) +f(drug response)
>
> first, I tried full omega block, and model was able to converge, but
> $COV stop failed.
>
> To me, this indicates that too many parameters in the model. The
> structure model is rather simple one, so I think probably too many Etas.
>
> I wonder is there a good principle of Eta reduction that I could
> implement here. Any good reference?
>
>

--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand
n.holford
mobile: +33 64 271-6369 (Apr 6-Jul 17 2009)
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford

Received on Wed Apr 15 2009 - 12:17:57 EDT

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