[R] nls, convergence and starting values

Patrick Giraudoux patrick.giraudoux at univ-fcomte.fr
Sat Mar 28 10:40:01 CET 2009

Patrick Burns a écrit :
> Patrick Giraudoux wrote:
>> Bert Gunter a écrit :
>>> Based on a simple scatterplot of pourcma vs  transat, a 4 parameter 
>>> logistic
>>> looks like wild overfitting, and that may be the source of your 
>>> problems.
>>> Given the huge scatter, a straight line is about as much as would seem
>>> sensible. I think this falls into the "Why ever would you want to do 
>>> such a
>>> thing?" category.
>>> -- Bert
>> Right, well, the general idea was just to show that the "straight 
>> line" was the best model indeed (in the other data sets, with model 
>> comparison, the logistic one was clearly shown to be the best... ). 
>> Can the fact that convergence cannot be obtained be an acceptable and 
>> sufficient reason to select the null model (the straight line) ?
> It is my experience that convergence problems are
> often encountered when the model makes little sense.
> I'm not so sure that non-convergence on its own is
> a good reason to reject  the model.  That is, to answer
> your specific question, I think it is acceptable but not
> sufficient.
> Patrick Burns
> patrick at burns-stat.com
> +44 (0)20 8525 0696
> http://www.burns-stat.com
> (home of "The R Inferno" and "A Guide for the Unwilling S User") 

OK. Thanks for this opinion. Actually I was sharing it intuitively but 
facing such situation for the first time, was quite unconfortable to 
make a decision (and still I am). We are touching epistemology...  and 
maybe a bit far from purely technical thus from the R list issues.

Tanks again, anyway,


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