[R] Logit reality check
Thomas Lumley
tlumley at u.washington.edu
Sun Sep 28 17:36:48 CEST 2003
On Sun, 28 Sep 2003, Andrew Criswell wrote:
> Hello all:
>
> I've been given the following data and have been asked to run a logit
> model using glm(). The variable, Y, is a proportion ranging from 0 to
> 1, X is a covariate. Without a base number of observations from which Y
> is computed as a proportion, I believe there is not sufficient information.
>
> If I try the model below, R seems to grumble with a complaint.
>
> glm(cbind(Y,1-Y) ~ X, family = binomial)
>
> non-integer counts in a binomial glm! in: eval(expr, envir, enclos)
>
> Am I correct to believe that more information is required?
Yes, probably.
If they are proportions without a well-defined denominator you may be able
to model them using family=quasi() and specifying the link and variance
function for a logistic regression model. You'd need to look at what the
variance function actually is, though.
McCullagh & Nelder's book has an example using proportions of leaf damage
that's a bit like this, although they end up using (mu(1-mu))^2 as the
variance function.
-thomas
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