Hi: One approach would be to use dlply() from the plyr package to generate the models and assign the results to a list, something like the following:
library(plyr) # function to run the GLM in each data subset - the argument is a generic data subset d gfun <- function(d) glm(Stems ~ Time, data = d, family = poisson) mlist <- dlply(stems139, .(SizeClass, Species), gfun) To see the result, try mlist[[1]] or summary(mlist[[1]]) to execute the print and summary methods on the first fitted model. Each output list object from glm() is a list component of mlist, so mlist is actually a list of lists. You can extract various pieces from mlist by using ldply() with a suitable extraction function or by use of the do.call/lapply combination. All of this is untested since no minimal example was provided per instructions in the Posting Guide... HTH, Dennis On Tue, Feb 8, 2011 at 11:54 AM, Christopher R. Dolanc <crdol...@ucdavis.edu > wrote: > 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 > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
