Hi all,

     I'm working with a sizable dataset that I'd like to summarize, but I
can't find a tool or function that will do quite what I'd like.  Basically,
I'd like to summarize the data by fully crossing three variables and getting
a count of the number of observations for every level of that 3-way
interaction.  For example, if factors A, B, and C each have 3 levels (all of
which were observed someplace in the dataset), I'd like to know how many
times A1, B1, and C1 co-occurred in the dataset.  Functions like aggregate
and summaryBy do a decent job when I sum a vector of ones of the same length
as the original dataset, but I'm getting stuck on the fact that neither will
return 0-count combinations of the three variables in question.  I
understand that this is a desirable outcome (if A1, B1, C2 didn't occur, it
shouldn't be counted and isn't), but I need to know both when these
combinations of factor did and did not occur.  I'm stuck on this one, and
would really appreciate any help.  Thanks in advance!

Dave Warren

PS A functional solution would be best; the original dataset contains about
2.3 million observations, so any looping is going to be very slow.

-- 
Post-doctoral Fellow
Neurology Department
University of Iowa Hospitals and Clinics
davideugenewarren@gmail.com

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