Re: Additive residual error
Siwei,
A final gradient of zero is not necessarily pathological. It just means that the fit cannot be improved by changing that parameter. As usual you need to decide if the estimate of the residual error parameter is plausible of not. The gradient cannot tell you that.
A zero additive error will always cause an error if you have a predicted conc of zero. This is because the likelihood involves a division by the residual error. If this is zero then the "computer says no < http://en.wikipedia.org/wiki/Carol_Beer >". I assume that is what you mean when you say "NM would not run".
Nick
Quoted reply history
On 12/02/2013 12:53 p.m., siwei Dai wrote:
> Hi, NM users:
>
> I have a typical 2-compartment model that can describe my data quite well, except the final gradient for the additive residual error is '0'. I therefore fix the additive residual error to '0', but then NM would not run. I tried different initial estimations for other parameters but the additive residual error seems to be the one that decide whether NM will run. Can anyone tell me why this would happen and how to solve it?
>
> Thank you very much in advance for your help.
>
> Siwei
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Nick Holford, Professor Clinical Pharmacology
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