[R] nlme problems

[Ken Beath]

I meant fitting not maximising, it is a nonlinear mixed effects  
model, with both fixed and random effects. My assumption is that for  
the function I am using the approximation approach used in nlme is  
not quite close enough, and nothing much that I can do, except for  
looking at starting values. I was hoping that someone would have  
other suggestions, so I will keep attempting to understand the  
control parameters.  I can add an extra parameter to the model and  
obtain a worse fit.

Ken

Dieter Menne 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
>