Message-ID: <4FE7BF52-E91C-4CD0-A428-828145918D3F@efs.mq.edu.au>
Date: 2005-12-19T09:11:23Z
From: Ken Beath
Subject: nlme problems
In-Reply-To: <s3a5f224.035@mail.efs.mq.edu.au>
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
>