[R] Contingency tables as data frames

[Kurt Hornik]

>>>>> presnell  writes:

> I'm teaching a categorical data analysis course this term, and a minor
> "problem" has resurfaced that I have often thought about before.  This
> applies equally to Splus I suppose, but my undergrads aren't using
> Splus.

> It seems natural to read/represent a contingency table as a data
> frame, with one column representing the cell counts (as in the example
> appended below (data taken from Agresti, "An Introduction to
> Categorical Data Analysis").  However, functions like ftable,
> mantelhaen.test, chisq.test, fisher.test, etc. don't work naturally
> with this representation, and instead require the user to first
> manipulate the data, say by using tapply to convert the data into an
> array.  This is not difficult of course, but it's one of those things
> that I'd rather not have to explain to students, who usually need to
> be focusing on other things.

> So, am I missing something obvious (not unlikely), or would it be a
> good idea to extend the methods/arguments of these functions to
> analyze/manipulate data represented in this way without any
> preprocessing by the user?  It seems that a "count" (or "weight" or
> "freq" or whatever) argument would do it in most cases.

> Funny, I can't help but wonder if the answer from those who have
> thought about this more deeply than I have might be "it's a can of
> worms".

Very good idea.

I think that S-PLUS crosstabs() can do that.

Let me think some more about this ...

-k
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