I'm having a hard time figuring out how to group results by certain
factors in R. I have data with the following headings:
[1] "Time" "Plot" "LatCat" "Elevation" "ElevCat"
"Aspect" "AspCat" "Slope"
[9] "SlopeCat" "Species" "SizeClass" "Stems"
and I'm trying to use a GLM to test differences in "Stems" for different
categories/factors - most importantly, I want to group things so that I
see results by "SizeClass" and then by "Species". This is pretty easy
in SAS using the "Group By" command, but in R, I haven't figured it out.
I've tried using the following code:
> stems139GLM <- glm(Stems ~ Time | SizeClass | Species,
family=poisson, data=stems139)
but R gives me this message:
Error in pmax(exp(eta), .Machine$double.eps) :
cannot mix 0-length vectors with others
In addition: Warning messages:
1: In Ops.factor(Time, SizeClass) : | not meaningful for factors
2: In Ops.factor(Time | SizeClass, Species) : | not meaningful for factors
I'd appreciate any help.
Thanks.
--
Christopher R. Dolanc
PhD Candidate
Ecology Graduate Group
University of California, Davis
Lab Phone: (530) 752-2644 (Barbour lab)un
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