# Re: distribution assumption of Eta in NONMEM

From: Nick Holford <n.holford>
Date: Fri, 28 May 2010 20:50:51 +0100

For estimation NONMEM estimates one parameter to describe the
distribution of random effects -- this is the variance (OMEGA) of the
distribution. Thus it makes no explicit assumption that the distribution
is normal. AFAIK any distribution has a variance.

For simulation NONMEM assumes all etas are normally distributed. If you
use OMEGA BLOCK(*) then the distribution is multivariate with
covariances but still normal.

Nick

Ethan Wu wrote:
> I could not find in the NONMEM help guide that explicitly mentioned a
> normal distribution is assumed, only it was clearly mentioned of
> assumption of mean of zero.
>
> ------------------------------------------------------------------------
> *From:* Serge Guzy <GUZY
> *To:* Ethan Wu <ethan.wu75
> *Sent:* Fri, May 28, 2010 1:25:24 PM
> *Subject:* RE: [NMusers] distribution assumption of Eta in NONMEM
>
> As far as I know, this is the assumption in most of the population
> programs like NONMEM, SADAPT, PDX-MC-PEM and SAEM. Therefore when you
> simulate, random values from a normal distribution are generated.
> However, you have the flexibility to use any transformation to create
> distributions for your model parameters that will depart from pure
> normality. For example, CL=theta(1)*exp(eta(1)) will generate a
> log-normal distribution for the clearance although the random deviates
> are all from the normal distribution.
>
> I am not sure how you can simulate data sets if you are using the non
> parametric option that is indeed available in NONMEM.
>
> Serge Guzy; Ph.D
>
> President, CEO, POP_PHARM
>
> www.poppharm.com <http://www.poppharm.com/>
>
>
>
>
>
>
>
>
>
> *From:* owner-nmusers
> [mailto:owner-nmusers
> *Sent:* Friday, May 28, 2010 9:08 AM
> *To:* nmusers
> *Subject:* [NMusers] distribution assumption of Eta in NONMEM
>
>
>
> Dear users,
>
> Is it true NONMEM dose not assume Eta a normal distribution?
>
> If it does not, I wonder what distribution it assumes? I guess this
> is critical when we do simulations.
>
> Thanks
>
>
>
>
>
>
> ------------------------------------------------------------------------
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--
Nick Holford, Professor Clinical Pharmacology
Dept Pharmacology & Clinical Pharmacology
University of Auckland,85 Park Rd,Private Bag 92019,Auckland,New Zealand
tel:+64(9)923-6730 fax:+64(9)373-7090 mobile:+64(21)46 23 53
email: n.holford
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford

Received on Fri May 28 2010 - 15:50:51 EDT

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