Re: 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 [EMAIL PROTECTED] tel:+64(9)373-7599x86730 fax:+64(9)373-7090 www.health.auckland.ac.nz/pharmacology/staff/nholford