[R] underflow of fisher.test result

(Ted Harding) Ted.Harding at manchester.ac.uk
Tue Oct 20 16:14:29 CEST 2009

On 20-Oct-09 13:34:49, Peng Yu wrote:
> fisher.test() gives a very small p-value, which is underflow on my
> machine. However, the log of it should not be underflow. I'm wondering
> if there is a way get log() of a small p-value. An approximation is
> acceptable in this case. Thank you!
>> fisher.test(rbind(c(10000,100000),c(100000,10000000)))$p.value
> [1] 0

I have not attempted an exact approach (though may do so later),
but the P-value in question is about 1e-15000 so (using log to
base e)

  log(P) approx = -33000

In such a case, P=0 is a pretty good approximation!

Which prompts the question: Why the interest in having the value
of such a very small number?

E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 20-Oct-09                                       Time: 15:14:26
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