[R] chisq.test probabilities method unclear

[(Ted Harding)]

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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