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RE: OMEGA BLOCK with mixture model?

From: e.krekels
Date: Mon, 04 May 2009 13:55:12 +0200

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-nmusers
n
> 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, the=
re
> seem to be two subpopulations in the exposure, let's say a large group wit=
h
> 'normal' and a second group with high exposure. I would like to identify t=
he
> 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 unreasonabl=
e
> 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
>
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> http://www.nycomed.com
>
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Received on Mon May 04 2009 - 07:55:12 EDT

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