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S Ellison S.Ellison at LGCGroup.com
Tue Jan 8 14:26:03 CET 2013

>I am using the nls function and it stops because the number of iterations
>exceeded 50, but i used the nls.control argument to allow for 500
>iterations. Do you have any idea why it's not working?

Not entirely. But I do see in the code for SSgompertz that SSgompertz calls nls _without_ specifying maxiter. So I suspect the error may be coming from the selfstart step which is ignoring your control argument.

You could check that by adding trace=TRUE to the nls call; I suspect you may see no iterations at all if the self-start fails before nls itself gets going. 

If that's so, the easiest way I can see round that (barring alrternative packages I've not heard of) is to copy the SSgompertz code to a text file (say 'SSgompertz' at the command line without () to see the code), create your own SSgompertz function from it, add a big maxiter in its internal call and use your version.

Failing that, use good starting parameters (usually I fool around with plots until something looks close enough to at least start) for a roll-your-own gompertz model.

S Ellison

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