[R] Fisher test problem

Thomas Lumley tlumley at u.washington.edu
Sun Mar 22 08:29:17 CET 2009

On Sat, 21 Mar 2009 markleeds at verizon.net wrote:

> by definition, the one tailed p-value has to be <= 0.5 so there is still
> something wrong with your OpenEpi calc.

I think that's a little strong.

Firstly, a one-tailed test in a pre-determined direction can have any p-value.

Secondly, even the smaller one-sided tail probability can be >0.5. The problem is discreteness.  It may be that 0.62 is the smaller of p(T>=t) and p(T<=t), and this turns out to be the case.  The other tail probability is 0.83.

OpenEpi should still be thresholding the p-value at 1, but that's just cosmetic. 


Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle

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