[R] nls, convergence and starting values

[Patrick Burns]

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")
>
> Patrick
>
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