[R] nls convergence problem

[Dieter Menne]

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
> GraphPad.
>

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:

http://www.menne-biomed.de/gastempt/gastempt1.html

> 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.

Dieter