[R] chisq.test probabilities method unclear
(Ted Harding)
Ted.Harding at nessie.mcc.ac.uk
Wed Dec 1 23:39:53 CET 2004
On 01-Dec-04 Alexander Keller wrote:
> Hi list,
>
> 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.
>
> i thought it might calculate relations, but it differs in the results
> (example: of typing p=c(1,1,1,1), p=c(6,6,6,6),
> p=c(0.25,0.25,0.25,0.25))
>
> can someone tell my about this chisq method, and perhaps show me an
> explaining example?
Experiment shows:
x<-c(6,6,6,6,6,6)
P<-c(6,6,6,6,6,6)
chisq.test(x,p=P)
Chi-squared test for given probabilities
data: x
X-squared = 1225, df = 5, p-value = < 2.2e-16
sum(((x-36*x)^2)/(36*6))
[1] 1225
that chisq.test(x,p=P) appears to calculate
sum( ((x - n*P)^2)/(n*P) )
whether the P sum to 1 or not (where n = sum(x)).
So you'd better make sure that they do!
Best wishes,
Ted.
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Date: 01-Dec-04 Time: 21:42:11
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