[R] nlme problems

[Dieter Menne]

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