[R] warnings associated with logistic regression

[Allan Strand]

Hi all,

This is as much as statistical/estimation question as an R-specific
one, but here goes.

I am trying to use logistic regression to predict suitability of
habitats for certain plant species.  The response variable is a binary 
one that indicates whether a particular species is found at a site on
the landscape.  The independent variables represent physical
characteristics of the landscape derived from a GIS.  A significant
proportion of the time I get the following warning messages from
glm():

> lr <- glm(known.v1~elevation+aspect+slope+energy15+energy166+aspect+accum+streams.buffered,family=binomial,data=siteframe)
Warning messages: 
1: Algorithm did not converge in: (if (is.empty.model(mt)) glm.fit.null else glm.fit)(x = X, y = Y,  
2: fitted probabilities numerically 0 or 1 occurred in: (if (is.empty.model(mt)) glm.fit.null else glm.fit)(x = X, y = Y,  

Now I can get the algorithm to converge (or at least not produce the
warning) by increasing the number of iterations, but that does not
affect the second warning.  A read of Hosmer and Lemeshow (1989) does not
provide much insight, so I thought that I would post the question  to
the list.

Any comments? Also, I'd be happy to email a dataset that exhibits this
behavior if anyone is curious enough.

Cheers,
A.

-- 
Allan Strand,   Biology    http://linum.cofc.edu
College of Charleston      Ph. (843) 953-8085
Charleston, SC 29424       Fax (843) 953-5453

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