[R] step() and stepAIC()
Prof Brian Ripley
ripley at stats.ox.ac.uk
Sun Oct 12 08:47:33 CEST 2008
On Sun, 12 Oct 2008, Murray Jorgensen wrote:
> The birth weight example from ?stepAIC in package MASS runs well as
> indeed it should.
> However when I change stepAIC() calls to step() calls I get warning
> messages that I don't understand, although the output is similar.
Why would you do this?
> Warning messages:
> 1: In model.response(m, "numeric") :
> using type="numeric" with a factor response will be ignored
> (and three more the same.)
I presume you did
birthwt.glm <- glm(low ~ ., family = binomial, data = bwt)
birthwt.step <- step(birthwt.glm, trace = FALSE)
birthwt.step2 <- step(birthwt.glm, ~ .^2 + I(scale(age)^2)
+ I(scale(lwt)^2), trace = FALSE)
and are referrring to the second call?
> Checked with 2.7.0 on Os X and 2.6.2 on Windows.
Those are not current versions. But the difference is that stepAIC uses
addterm and step uses add1, and this is a (harmless) infelicity in
> Cheers, Murray
> Dr Murray Jorgensen http://www.stats.waikato.ac.nz/Staff/maj.html
> Department of Statistics, University of Waikato, Hamilton, New Zealand
> Email: maj at waikato.ac.nz Fax 7 838 4155
> Phone +64 7 838 4773 wk Home +64 7 825 0441 Mobile 021 1395 862
> R-help at r-project.org mailing list
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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