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

Bálint Czúcz czucz at botanika.hu
Mon Aug 31 14:24:51 CEST 2009

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

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 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/

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