[R] questions please
Jason Turner
jasont at indigoindustrial.co.nz
Fri Oct 24 21:36:16 CEST 2003
Simone Panigada wrote:
...
> * I am running a glm to model animal abundance relating it to research
> effort and other variables. I have 10 years of data and I am treating year
> as a factor, R is comparing each year with the first year my data sheet,
> while I would like to compare each year with each other. Is there any
> function in R to do this?
...
Short answer:
If your model was glm(critters ~ year, ...), change this to
your.model <- glm(critters ~ year - 1,...)
In other words, drop the intercept if you don't want to compare to a
particular year. A model that defines coefficients for each year *and*
an overall intercept is over-defined; there's no additional information
given by the extra term.
You might also benefit from the package multcomp (which requires
mvtnorm) which has the function simint().
Long answer:
Get your hands on a copy of "Modern Applied Statistics with S" by
Venables and Ripley, and read chapters 6 and 7 rather thoroughly. This
goes into the reasons behind the choice of comparing one year to others.
Cheers
Jason
--
Indigo Industrial Controls Ltd.
http://www.indigoindustrial.co.nz
64-21-343-545
jasont at indigoindustrial.co.nz
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