[Rd] quick question about glm() example
Ben Bolker
bbolker at gmail.com
Wed May 29 19:58:03 CEST 2013
I don't have a copy of Dobson (1990) from which the glm.D93 example is
taken in example("glm"), but I'm strongly suspecting that these are
made-up data rather than real data; the means of the responses within
each treatment are _identical_ (equal to 16 2/3), so two of the
parameters are estimated as being zero (within machine tolerance). (At
this moment I don't understand why the means rather than the geometric
means being identical is what matters ...)
This therefore feels like a somewhat strange (i.e. non-generic)
example to use (although I know it's been that way for a long time).
Perhaps more importantly, the example illustrates the use of glm()
without a data= argument, which I think should not generally be
encouraged. I would prefer to see the example written as:
d.AD <- data.frame(
counts=c(18,17,15,20,10,20,25,13,12),
outcome=gl(3,1,9),
treatment=gl(3,3))
print(d.AD)
glm.D93 <- glm(counts ~ outcome + treatment, family = poisson(),
data=d.AD)
Thoughts?
Ben Bolker
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