[R] chisq.test probabilities method unclear

Rolf Turner rolf at math.unb.ca
Wed Dec 1 21:07:26 CET 2004

Alexander Keller wrote:

> i've got a question about the chisq.test function.  in the use of the
> "given probabilities" method  (p= ...), normally there should be
> typed in probabilities in the range of 0 to 1 with the absolute sum
> of 1.0 (r-help) But it is possible to use probabilities > than 1. or
> the sum <1.!  without any warning message
> Ok, now the question, what does r calcutate in these cases,
> this doesn't make sense in my (poor statistical) view.
> can someone tell my about this chisq method, and perhaps show me an 
> explaining example?

Take a look at the code; it just sets E <- n*p, and then calculates
sum((x-E)^2/E).  This sum is calculable even if p is not a vector of

And you are quite right; it ***doesn't*** make any sense in
those circumstances.

This might be termed a bug; of course only a silly user would supply
a p argument that wasn't a vector of probabilities .... but there are
a lot of silly users out there/here! :-)

(A p could arise from some other calculations where things could
go wrong in an unforseen way ....)

At the very least the code fails to practice ``safe statistical

IMHO there ought be a check in the code to make sure that the vector
p makes sense --- perhaps renormalizing it to sum to 1 (if all
entries are positive) along the lines of sample().

This would be very easy to write --- I'd volunteer, except that
I'm sure that the R core team would disdain my assistance.


				Rolf Turner
				rolf at math.unb.ca

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