NONMEM Users Network Archive

Hosted by Cognigen

Re: OMEGA selection

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


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
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.


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
mobile: +33 64 271-6369 (Apr 6-Jul 17 2009)

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

The NONMEM Users Network is maintained by ICON plc. Requests to subscribe to the network should be sent to:

Once subscribed, you may contribute to the discussion by emailing: