Re: Visual predictive check!

I prefer to do analysis in log-transformed variables thus avoiding this question entirely. Alternatively, you may try to create an error model that never returns negative values even in the non-transformed variables, but it is more complicated. One way to compare simulation results with the real data is to remove BQL values from the simulated data set, compare data above LLOQ, but then also compare percent of data that is BQL (in the real and simulated data set). With NM6, you can get expected percent below LLQ (1-PR_Y) using YLO functionality Leonid -------------------------------------- Leonid Gibiansky, Ph.D. President, QuantPharm LLC web: www.quantpharm.com e-mail: LGibiansky at quantpharm.com tel: (301) 767 5566 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