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Re: OMEGA BLOCK with mixture model?

From: Chenguang Wang <maurice.wang>
Date: Tue, 14 Apr 2009 17:51:19 +0200

Dear Nele,
To figure out the error of NONMEM, you can change the sequence of your
OMEGAs,
$OMEGA BLOCK(3)
0.1 ;CL1
0.01 0.1 ;F1
0 0.01 0.1 ;CL2
Make the corresponding change in the $PK block.


Regards,

Chenguang



2009/4/14 <nele.plock

>
> 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
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> D-78467 Konstanz, Germany
>
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>
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Received on Tue Apr 14 2009 - 11:51:19 EDT

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