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")