[Rd] Wierd nlm behaviour in 2.10.1 and 2.12.0 [Sec=Unclassified]

[Ravi Varadhan]

Hi Troy,

I think that your problem is poorly scaled.  You have variables that vary
over several orders of magnitude. This means that any small changes (due to
subtle differences between R versions) could cause a big difference in the
convergence behavior of the algorithms.  So, you are asking for trouble and
you got it.  I would scale the problem such that the parameters, function
and its gradient are essentially of the same magnitude.  

To paraphrase John Nash, who paraphrased Richard Varga, "It is all about
scaling".

Furthermore, I would specify the analytic gradient, which should be very
easy to derive.

Ravi.


-----Original Message-----
From: r-devel-bounces at r-project.org [mailto:r-devel-bounces at r-project.org]
On Behalf Of Troy Robertson
Sent: Wednesday, October 13, 2010 1:51 AM
To: 'r-devel at R-project.org'
Subject: [Rd] Wierd nlm behaviour in 2.10.1 and 2.12.0 [Sec=Unclassified]

Hi all,

When upgrading to 2.11.1 recently I noticed different results being produced
by my code.
After MUCH digging into my code I have finally narrowed it to a call to
nlm().
The problem actually occurs in 2.10.1 and 2.12.0.

This can be replicated by trying the code below in some different versions
of R:

FixedRemovals<-1836180125888
AbStageInitial<-2223033830403
Rates<- 0.3102445
myfunc<-function(rootM,Abund,Loss,OtherM)
     {(Loss-(rootM/(rootM+OtherM)*
      (1-exp(-(rootM+OtherM)))*
      Abund))^2}

nlm(myfunc
   ,0.001,print.level=0,fscale=0, gradtol=1E-10, stepmax = 100.0
   ,Loss=FixedRemovals
   ,Abund=AbStageInitial
   ,OtherM=Rates)

nlm(myfunc
   ,0.001,print.level=0,fscale=0, gradtol=1E-10, stepmax = 100.0
   ,Loss=FixedRemovals
   ,Abund=AbStageInitial
   ,OtherM=Rates)$estimate

Why would I be seeing a different value of 'estimate' (or any of the other
returned list items)?
Originally I thought it was 2.11.1 that was giving me the grief with
different output but now I see that in 2.11.1 I get the same value to each
returned list item both ways.
Can anyone explain this?

Thanks

Troy
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