Andreas,
You've raised an important, but sometimes overlooked, point about
model checking using simulation-based methods. As Andrew Gelman points =
out in the reference below, when comparing simulated vs observed
values you need to compare apples to apples. Either incorporate a
model for the missing data in the simulation and compare the subset of =
non-missing data only, or impute the missing observed data and compare =
the complete (e.g. no-missing data) data sets.
Gelman et al. Multiple Imputation for Model Checking: Completed-Data
Plots with Missing and Latent Data. Biometrics 61, 74–85 , March 2005
Marc
Marc R. Gastonguay, Ph.D.
President & CEO, Metrum Research Group LLC [www.metrumrg.com]
Scientific Director, Metrum Institute [www.metruminstitute.org]
Direct: 860-670-0744 Main: 860-735-7043
Email: marcg_at_metrumrg.com
On May 23, 2008, at 6:22 AM, 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
>
Received on Fri May 23 2008 - 07:34:32 EDT
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