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From: Stephen Duffull <stephen.duffull>
Date: Thu, 26 Feb 2009 07:33:15 +1300


> We have a dataset with as many dosing (amount and length of
> infusion) as patients. Once the final model was defined, I
> have performed a vpc. However, because the dosing are very
> different between patients, is it relevant to perform vpc or
> shall we compute npc or ppc?
> Can somebody explain the basic difference between vpc, npc
> and ppc and when shall we used one or the other?

Despite how it sounds this is not really a simple question.

Mostly the purpose of all of these techniques is to assess how well the mod=
el describes the data. This can be achieved visually and numerically. If =
you want your method to have "diagnostic" properties, i.e. an ability to de=
termine where the model may fail, then visual types of checks tend to be mo=
re informative. Numerical types of checks really give you an overall feeli=
ng of whether your model fits the data but don't often allow you to determi=
ne where in particular the model might fail.

Numerical checks include PPC and NPDE (and others). PPC is really a Bayesi=
an construct as it checks the posterior distributions of your parameters an=
d hence isn't naturally something that would be performed in an MLE framewo=
rk. However there have been many examples where PPCs have been performed i=
n NONMEM (publications on this have appeared in JPKPD). PPC is generally u=
sed to test a specific (important) feature of the data (hence is generally =
not diagnostic for the whole model). NPDE provides a more general numerica=
l description of agreement of model and data, but when the statistic is tes=
ted it seems to reject most model (hence is not diagnostic).

Despite the apparent division into visual and numerical there is no reason =
why a "VPC" couldn't be expressed numerically as a numerical predictive che=
ck and why PPC or NPDE style techniques could not be expressed graphically.=
  We have recently produced examples of visual versions of PPC as a form of=
 visual predictive check for situations similar to what you have described =
where traditional VPCs don't work well (note we did this in WinBUGS).

Professor Stephen Duffull
Chair of Clinical Pharmacy
School of Pharmacy
University of Otago
PO Box 913 Dunedin
New Zealand
E: stephen.duffull
P: +64 3 479 5044
F: +64 3 479 7034

Design software:

Received on Wed Feb 25 2009 - 13:33:15 EST

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