[R] GAM for non-integer proportions

Gavin Simpson gavin.simpson at ucl.ac.uk
Tue Feb 23 09:32:10 CET 2010

On Mon, 2010-02-15 at 01:33 -0500, Julian Burgos wrote:
> Dear list,
> Im using the mgcv package to model the proportion by weight of certain prey
> on the stomach content of a predator.  This proportion is the ratio of two
> weights (prey weight over stomach weight), and ranges between 0 and 1.  The
> variance is low when proportion is close to 0 and 1, and higher at
> intermediate values.
> It seems that the best way to go is to model this using the "quasi" family
> with a logit link and a mu(1-mu) variance.  Or I am missing something
> obvious?  I will be thankful for any input.

Could you include the stomach weight as an offset term in your formula?
IIRC Simon Wood has an example in his GAM book where he uses offset(...)
to standardise different net sizes used to capture a count response.
That way you could model the prey weight directly.

Just a thought.


> All the best,
> Julian
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 Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
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