NONMEM Users Network Archive

Hosted by Cognigen

RE: VPC, NPC or PPC?

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

Nicolas

> 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).

Steve
--
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: www.winpopt.com

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

The NONMEM Users Network is maintained by ICON plc. Requests to subscribe to the network should be sent to: nmusers-request@iconplc.com.

Once subscribed, you may contribute to the discussion by emailing: nmusers@globomaxnm.com.