RE: omega matrix - friendly suggestion
Dear Pavel,
I certainly feel your pain but you have to be careful how you fix certain
elements in Omega to ensure that you have a valid positive definite
covariance matrix. The starting values in your $OMEGA block do not give
rise to a valid covariance matrix. Note in particular that the covariance
between ETA3 and ETA4 is too large relative to the variances for ETA3 and
ETA4 such that the correlation is greater than 1.0, i.e.,
Corr(ETA3,ETA4) = 0.03/[SQRT(0.0166)*SQRT(0.0166)]=1.807 > 1
You also have the same problem for Corr(ETA1,ETA3) > 1.
Ken
Kenneth G. Kowalski
President & CEO
A2PG - Ann Arbor Pharmacometrics Group, Inc.
110 Miller Ave., Garden Suite
Ann Arbor, MI 48104
Work: 734-274-8255
Cell: 248-207-5082
Fax: 734-913-0230
<mailto:[email protected]> [email protected]
http://www.a2pg.com www.a2pg.com
Quoted reply history
From: [email protected] [mailto:[email protected]] On
Behalf Of [email protected]
Sent: Tuesday, June 12, 2012 10:24 AM
To: [email protected]
Subject: [NMusers] omega matrix - friendly suggestion
Hello NONMEM Community,
Sometimes it takes more time to choose the best omega matrix than to develop
a PD model. Selecting the omega matric is a tedious, time consuming and
less creative part of the model development. I hope you feel my pain. Will
it be helpful to rewrite the NONMEM software so that any element of the
omega matrix can be fixed to any value? It may look like this:
$OMEGA BLOCK (4) 3.60E-02 FIX
0.01 3.23E-02
0.03 0 FIX 1.66E-02
0.01 0 FIX 0.03 FIX 1.66E-02
This change can make many NONMEM users happy.
Thanks!
Pavel