[R] nls convergence problem
dieter.menne at menne-biomed.de
Thu Aug 17 19:38:35 CEST 2006
Yours truly <dieter.menne <at> menne-biomed.de> writes:
> Recently, a colleague fitted gastric emptying
> curves using GraphPad, with 100% success, and
> nls failed for one third of these. When we
> checked GraphPads output more closely, some of
> the coefficients looked like 2.1 with a confidence
> interval in the range -27128 ... 314141. Nobody
> forces you to look at these, though, when using
Since my comment has stirred a bit of an uproar, I should add
that this is not the fault of GraphPad, but that most non-linear
fitting programs, including those in the big SXXX, give the same
results. Harvey Motulsky from Graphpad/Prism informed me that
they were going to add special tests in the new versions. Their
pdf-manual on nonlinear fitting is worth a look anyway if
Bates/Watts is over your head.
And if anyone want to see sample data:
> I only wish nls were a little bit more polite in telling me what went wrong.
I stand corrected:
I only wish nls were less politically correct, but rather inform me
WHY it faltered.
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