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Re: More Levels of Random Effects

From: Paul Hutson <prhutson>
Date: Fri, 17 Oct 2008 09:18:11 -0500
Please don't forget us Scots.

Leonid Gibiansky wrote:
e-mail: LGibiansky at
tel:    (301) 767 5566

Nick Holford wrote:

I dont understand what you mean by "we lose predictive power of the model: we do not know what will be
the variability on the next occasion.".

Or are you concerned about the situation where you have say 3 occasions and the IOV seems to be different on each occasion but you now want to predict the IOV for a future study on the 4th occasion?

I agree it is hard to extrapolate to future occasions but this seems to be just like any other non-ordered categorical covariate - e.g. if we see differences between English, Irish and Welsh what difference would you expect for Russians? :-)


Leonid Gibiansky wrote:
Hi Xia, Nick
Technically, one can use different variances on different occasions but
then we loose predictive power of the model: we do not know what will be
the variability on the next occasion. One can use occasion-dependent IOV
variance to check for trends (for example, to investigate the time
dependence of the IOV variability, or to check whether the first
occasion (e.g., after the first dose of a long-term study) is for some
reasons different from the others) but the final model should have some
condition that specifies the relations of IOV variances at different
occasion (SAME being the simplest, most reasonable and the most-often
used option).


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

Nick Holford wrote:

There is no requirement to use the SAME option. However, it is a reasonable model for IOV that it has the same variability on each occasion.

If you dont use the SAME option then you just need to estimate an extra OMEGA parameter for each occasion you dont use SAME. You can test if the SAME assumption is supported by your data or not by comparing models with and without SAME.


PS Your computer clock seems to be more than 2 years out of date. Your email claimed it was sent in 17 Jan 2006.

Xia Li wrote:
Dear All,
Do we have to assume the variability between all occasions are the same when
we estimate IOV? What will happen if I don't use the 'same' constrain in the
$OMEGA BLOCK statement? Any input will be appreciated.


Xia Li

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Paul R. Hutson, Pharm.D.

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UW School of Pharmacy

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Received on Fri Oct 17 2008 - 10:18:11 EDT

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