medians of the simulation in VPC are systematically below the medians of the observations when the model fit looks fine.

Dear All, I have built a non-compartment model, in which the medians of IPREDs and Observations are very close. But in VPC plot, the medians of simulations are systematically below the medians of observations Any idea why it happens? Thanks. Shirley Notice: This e-mail message, together with any attachments, contains information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New Jersey, USA 08889), and/or its affiliates Direct contact information for affiliates is available at http://www.merck.com/contact/contacts.html) that may be confidential, proprietary copyrighted and/or legally privileged. It is intended solely for the use of the individual or entity named on this message. If you are not the intended recipient, and have received this message in error, please notify us immediately by reply e-mail and then delete it from your system.