[R] nlme problems

Spencer Graves spencer.graves at pdf.com
Mon Dec 19 17:42:47 CET 2005

	  Are you familiar with Pinheiro and Bates (2000) Mixed-Effects Models 
in S and S-Plus (Springer)?  I suspect that book, especially the latter 
half, might contain the information you seek.

	  spencer graves
p.s.  PLEASE do read the posting guide! 
"www.R-project.org/posting-guide.html".  Anecdotal evidence suggests 
that posts more consistent with that guide tend to receive quicker, more 
useful replies.

Ken Beath wrote:

> 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 ?
>>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.
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Spencer Graves, PhD
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