NONMEM Users Network Archive

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Re: Very small P-Value for ETABAR

From: Leonid Gibiansky <LGibiansky>
Date: Thu, 13 Nov 2008 12:12:43 -0500

ETABAR p-value could be a useful quick check of the results, but it
should not be used to replace the graphical evaluation of the model.
Graphical evaluation should always include the histograms of the random
effects, QQ plot of the random effects versus standard normal
distribution, scatter plot matrix that visualizes correlation of the
random effects, and plots of random effects versus all important (or all
available) continuous and categorical covariates (as box-plots for
categorical). These visual checks are much more powerful tools to detect
a problem than ETABAR p-values. If they are OK than I would not worry
about ETABAR. However, small ETABAR p-value often hints that the ETA
distribution is not symmetric, or has outliers (non-symmetric long tails).

Leonid Gibiansky, Ph.D.
President, QuantPharm LLC
e-mail: LGibiansky at
tel: (301) 767 5566

Jian Xu wrote:
> A few years back, there was a discussion on the P-value for ETABAR.
> However, I am not sure how to appropriately handle very small P-value(s)
> for ETABAR situation during the development of a model.
> I need some clarifications to a few questions:
> 1: Should we just ignore this small P-Value warning?
> 2: Can we change IIV model to avoid small P-value for ETABAR? Or any
> other suggestions?
> 3: Does NONMEM make any assumptions on ETA distribution?
> This P-value for ETABAR really bugs me a lot. I look forward to seeing
> some input.
> Thank you and I appreicate your time and help.
> Jian
Received on Thu Nov 13 2008 - 12:12:43 EST

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