Hi All-

Thanks for your quick responses.  I was looking for unique instances, so
Jim's and Daniel's suggestions got the job done.  Using "length" alone
didn't discriminate between multiple occurrences of the same species and
multiple species.

I do have one followup question- my full data set (not the example data) has
a number of NAs in the SppID column, and [r] is currently counting the NAs
as species occurrences.  Using Jim's code, I tried:

>tapply(SppID, PlotID, function(Trees, na.rm=T) length(unique(Trees,
na.rm=T)))

and alternately:

>tapply(SppID, PlotID, function(Trees) length(unique(Trees)), na.rm=T)

which doesn't seem to convince [r] to ignore the NAs.  What am I doing
wrong?

Thanks,

Ian

	[[alternative HTML version deleted]]


