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RE: OMEGA BLOCK with mixture model?

From: Elodie Plan <elodie.plan>
Date: Tue, 14 Apr 2009 17:40:58 +0200

Hi Nele,


About the technical issue, just change the order of your etas (2 and 3) in
order to have the 0 FIX at the place of cov1-3, this gives you what is
called a band matrix.

Hope it will help!


Best regards,



            Elodie L. Plan, PharmD, MSc,



Department of Pharmaceutical Biosciences,
  Faculty of Pharmacy, Uppsala University

      Box 591 - 751 24 Uppsala - SWEDEN

Office +46 18 4714385 - Fax +46 18 4714003


From: owner-nmusers
Behalf Of nele.plock
Sent: Tuesday, April 14, 2009 5:09 PM
To: nmusers
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:
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

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.plock

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Received on Tue Apr 14 2009 - 11:40:58 EDT

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