Visual predictive check!

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