[R] nls convergence problem

[Dieter Menne]

nls not converging for zero-noise cases

 <Setzer.Woodrow <at> epamail.epa.gov> writes:

> 
> No doubt Doug Bates would gladly accept patches ... .
> 

The zero-noise case is irrlevant in practice, but quite often I have uttered
&/(!! (vituperation filter on) when nls did not converge with real data. The
dreaded "min step reduced...". And yet, I found that nls is damned right not to
behave nicely in many cases. 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.

I only wish nls were a little bit more polite in telling me what went wrong.

Dieter