Dear NMusers,
I have a question also related to omega blocks. Would it be okay to
have two such blocks?
In my model for instance correlation between eta 1, 2, and 3 seem
biologically plausible, as is a correlation between eta 4 and 5.
Correlations between 1, 2, or 3 and eta 4 or 5 is highly unlikely.
Could this be coded as follows (I used arbitrary initial estimates for
the omegas):
$OMEGA BLOCK(3)
0.1 ;eta 1
0.1 0.1 ;eta2
0.1 0.1 0.1 ;eta3
$OMEGA BLOCK(2)
0.1 ;eta4
0.1 0.1 ;eta5
Thank you for your input,
Elke Krekels
From: owner-nmus...@globomaxnm.com [mailto:owner-nmus...@globomaxnm.com] On
Behalf Of nele.pl...@nycomed.com
Sent: Tuesday, April 14, 2009 5:09 PM
To: nmusers@globomaxnm.com
Subject: [NMusers] OMEGA BLOCK with mixture model?
Dear all,
I am trying to fit a PK model to oral data. In the data, we observed two
things: First, CL seems to be negatively correlated with F1. Secondly, there
seem to be two subpopulations in the exposure, let's say a large group with
'normal' and a second group with high exposure. I would like to identify the
subpopulations using a mixture model, but keep the correlation between CL
and F1. Now I ran into problems when coding the $OMEGA BLOCK.
I figured the block to be something like:
$OMEGA BLOCK(3)
0.1 ;CL1
0 FIX 0.1 ;CL2
0.01 0.01 0.1 ;F1
The error message that appears is:
a covariance is zero, but the block is not a band matrix
I assume that this means that I am not allowed to fix the correlation
between the two clearance-omegas to zero. However, it would be unreasonable
to allow a correlation, because the omegas belong to different
subpopulations, so there can't be a correlation. On the other hand, I did
not include subpopulations for F1, so how can I keep this correlation to
both CL-subgroups?
Any thoughts on this would be highly appreciated!
Best wishes
Nele
______________________________________________________________
Dr. Nele Plock
Pharmacometrics -- Modeling and Simulation
Nycomed GmbH
Byk-Gulden-Str. 2
D-78467 Konstanz, Germany
Fon: (+49) 7531 / 84 - 4759
Fax: (+49) 7531 / 84 - 94759
mailto: nele.pl...@nycomed.com
http://www.nycomed.com
County Court: Freiburg, Commercial Register HRB 701257
Chairman Supervisory Board: Charles Depasse
Management Board: Dr. Barthold Piening, Gilbert Rademacher, Dr. Anders
Ullman
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