nls, convergence and starting values
Patrick Giraudoux wrote:
Bert Gunter a ?crit :
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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