[R] warning associated with Logistic Regression

[Guillem Chust]

Hi All,

When I tried to do logistic regression (with high maximum number of
iterations) I got the following warning message

Warning message:
fitted probabilities numerically 0 or 1 occurred in: (if
(is.empty.model(mt)) glm.fit.null else glm.fit)(x = X, y = Y,

As I checked from the Archive R-Help mails, it seems that this happens when
the dataset exhibits complete separation. However, p-values tend to 1, and
residual deviance tends to 0. My questions then is:
-Is the converged model correct? or
-Can I limit the number of iterations in order to avoid this warning? If I
do so, I’ve checked that the model selected by step can diverge in some
cases (I use 10 different presence-absence datasets and 18 explanatory
variables), and when I validate the model with independent data, the new
model is slightly more powerful (in the most part of cases).

Thanks in advance,

Guillem Chust

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Guillem Chust 				       chust at cict.fr

Laboratoire Evolution et Diversité Biologique, UMR 5174 CNRS/UPS
UPS Toulouse III, batiment IVR3
118, route de Narbonne  - 31062 Toulouse Cedex 4, France
Tel 33 (0)5 61 55 67 58          Fax 33 (0)5 61 55 73 27
http://www.edb.ups-tlse.fr/