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Re: OMEGA BLOCK with mixture model?

From: Leonid Gibiansky <LGibiansky>
Date: Tue, 14 Apr 2009 11:59:44 -0400

This should do the trick (rename ETAs):

0.1 ;CL1
0.01 0.1 ;F1
0 0.01 0.1 ;CL2

(do not FIX anything)

Although I am not sure whether you need to estimate F1 for oral data
(without IV). You could try to use ETA on V instead of ETA on F1.


Leonid Gibiansky, Ph.D.
President, QuantPharm LLC
e-mail: LGibiansky at
tel: (301) 767 5566

> 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
> 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
> Nele
> ______________________________________________________________
> Dr. Nele Plock
> Pharmacometrics -- Modeling and Simulation
> Nycomed GmbH
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> D-78467 Konstanz, Germany
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Received on Tue Apr 14 2009 - 11:59:44 EDT

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