Re: [R] nlme problems

From: Dieter Menne <dieter.menne_at_menne-biomed.de>
Date: Sun 18 Dec 2005 - 20:12:53 EST

Ken Beath <kbeath <at> efs.mq.edu.au> writes:

>
> I'm maximising a reasonably complex function using nlme (version
> 3.1-65, have also tried 3.1-66) and am having trouble with fixed
> parameter estimates slightly away from the maximum of the log
> likelihood. I have profiled the log likelihood and it is a parabola
> but with sum dips. Interestingly changing the parameterisation moves
> the dips around slightly. Unfortunately the PNLS step is finding a
> maximum at the dips rather than the mle. I have tried using starting
> values for the fixed parameters without change. Any ideas ?

Ken,

you should not use nlme for "maximising a complex function", because it's a rather specialized tool for mixed-model statistical analysis. Try to use optim directly, which has quite a few methods to choose from, and one of them might work for your problem.

Dieter



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