Dear R-help folks,
Can someone guide me to a source where I can learn more about the / operator
in model formulae? I found a reference to it in Venables and Ripley's MASS,
p. 142, where it says, in reference to ANCOVA: "Terms of the form a/x,
where a is a factor, are best thought of as 'separate regression models of
type 1 + x within the levels of a.'..."
This seems very appropriate to my analysis, where I am doing an ANCOVA of
tree growth as a function of tree height (ht - continuous) separately for
three species (spp) and three light levels (lt). The problem is, I'm not
exactly sure how to interpret the results of this model specification, and I
can't find any other references to it despite doing searches in the help
pages, Google, Jonathon Baron's R site search, and other text books such as
Dalgaard or Crawley.
I've fit the model as: growth ~ spp*lt / ht -1, and compared the results to
growth ~ spp*lt*ht.
The coefficients are equivalent in both models, producing identical plots of
predicted values. but the terms that appear in the summary table differ, as
do the P-values, and I'm not sure how to interpret them. In the more
standard model (growth ~ spp*lt*ht), I am clear on the fact that each level
of the main effects is compared to the lowest level of that main effect
(when using contr.treatment). But the alternative model (growth ~spp*lt /
ht -1) contains all three levels of the main effect, spp.
I'm not sure this is at all clear, but I am happy to try to clarify it.
Thanks much in advance,
Matt Landis
R. Matthew Landis, Ph.D.
Dept. Biology
Middlebury College
Middlebury VT 05753
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