Re: [NMusers] Visual predictive check!

Andreas,

There are many ways to create a VPC (come to PAGE this year and hear
about them from Mats Karlsson and myself).

In the specific case of using a VPC to simulate both the between
subject variability and the residual unidentified variability for
comparison with the observations then I would apply the same truncation
rules used to create the model. If you have a LLOQ defined and you are
in the unfortunate situation of not being able to use values less than
LLOQ then I would suggest you truncate the simulations.

If you have honestly reported measurements (i.e. without omitting
measurements less than LLOQ) then I would not truncate the simulated
values -- I would even accept negative simulated values because if your
residual error model was correct then there would be negative measured
values.

Best wishes,

Nick

andreas lindauer wrote:
>
> Dear NMusers,
>
>
>
> I have a question regarding simulations for a VPC. When a combined
> residual error is used it happens that for low concentrations negative
> values are simulated. While this is statistically correct, I wonder if
> it is correct to use these results for the VPC because the
> distribution of the observed low concentrations is truncated by the
> LLOQ. So the VPC might suggest model misspecification for lower
> concentrations. Further, when one wants to use the model for clinical
> trial simulation should then the negative (BQL) values be omitted
> because they would never appear in reality?
>
> I would like to know how the more experienced NMusers handle this.
>
> Thanks in advance, Andreas.
>
>
>
>
>
> ____________________________
>
>
>
> Andreas Lindauer
>
>
>
> University of Bonn
>
> Department of Clinical Pharmacy
>
> An der Immenburg 4
>
> D-53121 Bonn
>
>
>
> phone:+49 228 73 5781
>
> fax: +49 228 73 9757
>
>
>

--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand
n.holford_at_auckland.ac.nz tel:+64(9)373-7599x86730 fax:+64(9)373-7090
www.health.auckland.ac.nz/pharmacology/staff/nholford


Received on Fri May 23 2008 - 07:26:11 EDT