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RE: nonmem question regarding condition number

From: Denney, William S. <William.S.Denney>
Date: Thu, 24 Mar 2016 10:52:59 +0000

Hi Laureen,

Large condition numbers (typically interpreted as >1000) indicate that two =
or more parameters in the model are highly correlated in their covariance a=
nd that the model parameters are difficult to identify. Given your descrip=
tion below, I would not suggest using the 3-compartment model (even with it=
s 180 point reduction in OFV) because two of your parameters are so highly =
correlated that the condition number is reaching the level of computational=
 precision (2E16 is very large for a condition number). I'd also look at t=
he NONMEM output to see if the 3-compartment model had any errors around th=
e convergence.



From: owner-nmusers
 Behalf Of L.A. ten Berg - Lammers
Sent: Thursday, March 24, 2016 6:10 AM
To: 'nmusers
Subject: [NMusers] nonmem question regarding condition number

Dear nmusers,
I am currently analyzing PK profiles after a single iv adminstration. Volun=
teers were sampled frequently: 12 samples per curve.
For the structural model 2- and a 3-compartment models are compared. The CW=
RES vs time plot of the 2-compartment shows the typical wave, indicative fo=
r a 3-compartment model. Introduction of a third compartment eliminated the=
 wave, OFV decreased with 180 points, precision of the estimates did change=
 notably, but the condition number increased from 998 to 2.0*E16.
Does anyone know how to interpret this large increase in condition number?
Best wishes,
Laureen ten Berg

[Beschrijving: cid:image001.gif

Laureen ten Berg-Lammers

    Hospital Pharmacist

Academisch Medisch Centrum
Universiteit van Amsterdam

Meibergdreef 9 | 1105 AZ Amsterdam | Room: M01-224
Tel: +31(0)20-5662726 | Pager *62726 | l.a.tenberg-lammers


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Received on Thu Mar 24 2016 - 06:52:59 EDT

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