Re: medians of the simulation in VPC are systematically below the medians of the observations when the model fit looks fine.
Shirley,
IPREDs are notoriously unreliable for model evaluation (Holford 2005) especially if there is more than 25% shrinkage (Karlsson & Savic 2007).
The VPC, although not without pitfalls, is more likely to be showing you how well your model is performing (Bergstrand et al. 2011). If there is a systematic under-prediction of the observations then you should consider reasons why this might be and try another model to test your ideas.
I am afraid I have no idea what you mean by a 'non-compartment model' in this context.
Nick
Holford NHG. The visual predictive check – superiority to standard diagnostic (Rorschach) plots [www.page-meeting.org/?abstract=738]. PAGE. 2005;14. Karlsson MO, Savic RM. Diagnosing model diagnostics. Clin Pharmacol Ther. 2007;82(1):17-20. Bergstrand M, Hooker AC, Wallin JE, Karlsson MO. Prediction-corrected visual predictive checks for diagnosing nonlinear mixed-effects models. AAPS J. 2011;13(2):143-51.
Quoted reply history
On 4/02/2012 11:48 a.m., Hou, Xiaoli Shirley wrote:
> Dear All,
>
> I have built a non-compartment model, in which the medians of IPREDs and Observations are very close. But in VPCplot, the medians of simulations are systematically below the mediansof observations
>
> Any idea why it happens?
>
> Thanks.
>
> Shirley
>
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