From: Alison_Carter@eisai.com>
Subject: [NMusers]
Date: Tuesday, July 30, 2002 12:23 PM
Dear NONMEM users,
I am having a frustrating time trying to fit a simple 2-compartment model
to population data. I can fit the individual subjects with 2-compartments
fairly easily, but when I try them together, I get the message:
R MATRIX ALGORITHMICALLY NON-POSITIVE-SEMIDEFINITE
BUT NONSINGULAR COVARIANCE STEP ABORTED
I know from the manual that this means the apparent minimum might be
a saddle point and that I should not trust the final estimates. My
problem is what to do to correct the situation. I have tried various error models and
ranges of initial estimates, but I am out of ideas. Does anyone have any
suggestions? Does this indicate the data might be better fit with a
1-compartment model?
Thanks very much!
Alison Carter, PhD
(no subject)
From: "Venkatesh Atul Bhattaram"
Subject: [NMusers]
Date: Tue, 30 Jul 2002 13:01:09
Hello Alison
This could be due to overparameterization. I would check the estimates of
eta and their correlations and try to minimize them in the model. I found
this to work well. Also did you try other estimation methods.
Venkatesh Atul Bhattaram
Post-doctoral Fellow
University of Florida
Gainesville-32610