[R-sig-eco] glm for ratio [0,1] data

[Bálint Czúcz]

Dear List,

does anyone know a good way to perform GLM on ratio data (i.e. data
between 0 and 1)? Binomial GLM is quite straightforward to use if you
have integer numbers for successes/failures. But how to proceed if you
only have the ratio? This can occur in a multitude of ways, e.g the
response variable is the estimated cover of a species, percentage of
canopy lost, etc.

One solution I know about is to try to transform such responses to
normal with the arcsine-squarroot transformation, and use lm on the
transformed response -- e.g. Crawley (2007, The R Book, p. 570.)
explicitely suggests this strategy.

But I would still be interested if there is a glm approach that could
be used with the untransformed data. After hours spent with searching
for literature on such a glm, I couldn't find any. Do you know of
some?

I would also be interested what happens if I just proceed with a
binomial glm with the response being between [0,1] and weights left to
1. I know glm() will throw a warning -- but it also produces an
output. Can this output contain some valid, interpretable results, or
is it completely bullshit because of the violation of the assumptions?

Thank you!
Bálint


--
Bálint Czúcz
Institute of Ecology and Botany of the Hungarian Academy of Sciences
H-2163 Vácrátót, Alkotmány u. 2-4. HUNGARY
Tel: +36 28 360122/137  +36 70 7034692
magyar nyelvű blog: http://atermeszettorvenye.blogspot.com/