[R] logistic regression model specification

Dylan Beaudette
2007-11-13T21:59:34Z

Hi,

I have setup a simple logistic regression model with the glm() function, with
the follow formula:

y ~ a + b

where:
'a' is a continuous variable stratified by
the levels of 'b'


Looking over the manual for model specification, it seems that coefficients
for unordered factors are given 'against' the first level of that
factor.
This makes for difficult interpretation when using factor 'b' as a
stratifying model term.

Setting up the model, minus the intercept term, gives me what appear to be
more meaningful coefficients. However, I am not sure if I am interpreting the
results from a linear model without an intercept term. Model predictions from
both specifications (with and without an intercept term) are nearly identical
(different by about 1E-16 in probability space).

Are there any gotchas to look out for when removing the intercept term from
such a model?

Any guidance would be greatly appreciated.

Cheers,

--
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341