[R] logit regression, test among groups

[Martin Henry H. Stevens]

Dear all:
My logistic regression model includes one qualitative and one quantitative 
predictor variable,

aes <- glm(p.a ~ spp * log(light), family=binomial(link=logit)),

where spp is abundance of 3 species and light is subcanopy light 
availability varying from 0  1.

  I want to test differences among levels of the quantitative variable at a 
value of x other than the current log(light)=0.  I tried using 
I(log(light+.5)) but this caused serious lack of fit (AIC went from 1574 to 
1656, and diagnostic plots demonstrated poor fit). How would create a model 
statement such that I test differences among groups at log(light)=approx. 2?
Many thanks.

Martin Henry H. Stevens, Assistant Professor
338 Pearson Hall
Botany Department
Miami University
Oxford, OH 45056

Office: (513) 529-4206
Lab: (513) 529-4262
FAX: (513) 529-4243
http://www.muohio.edu/~botcwis/bot/henry.html

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._