[R] Confidence Limits for Non-Linear Regression

Prof Brian D Ripley ripley at stats.ox.ac.uk
Fri Jan 12 08:50:37 CET 2001

On Fri, 12 Jan 2001, Dieter Menne wrote:

> Dear friends of nls,
> I have tried confint.nls on a few logistic regression fit; all curves are
> well-behaved. Only in 1 out of 3 cases, the method converged, it stopped on the
> other cases with "singular matrix" or "number of iterations exceeded". I have
> put in the intermediate step by computing the profiles first and manipulating
> alphamax and delta.t, but I could not find a compromise that worked.
> Anybody can give me a hint?

Probably your problem is too hard for profile.nls.  I don't see that this
is much to do with confint.nls:  that calls profile[.nls] and the
latter is failing.  `the method' that is not converging is nls as called by
profile[.nls], I think.

I had thought if you turn tracing on, you would see the area in parameter
space where profile.nls gets in trouble, but profiling seems to turn it
off.  You could try increasing maxiter in nls.control.

Incidentally to Doug Bates: the `trace' argument of nls is missing from the
described arguments.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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