RE: Visual predictive check!
Andreas,
You might want to look at the reference below. Another way of comparing
apples to apples is to present the VPC while also visualizing the
percentage of expected but missing data (e.g. BQL data).
Post TM, Freijer JI, Ploeger BA, Danhof M. Extensions to the Visual
Predictive Check to facilitate model performance evaluation. J
Pharmacokinet Pharmacodyn. 2008 Apr;35(2):185-202
Teun
Teun Post, PharmD
Clinical Pharmacology and Kinetics - PK-PD - M&S
PK-PD Scientist
N.V. Organon, a part of Schering-Plough Corporation
KM2521 - PO Box 20 - 5340 BH - Oss - The Netherlands
T: +31 412 662782
F: +31 412 662506
_____
Quoted reply history
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of Gastonguay, Marc
Sent: Friday, 23 May, 2008 13:35
To: andreas lindauer
Cc: [email protected]
Subject: Re: [NMusers] Visual predictive check!
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: [EMAIL PROTECTED]
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
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