[R] underflow of fisher.test result

[Peng Yu]

On Tue, Oct 20, 2009 at 9:14 AM, Ted Harding
<Ted.Harding at manchester.ac.uk> wrote:
> 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?

This is just an example showing the result is underflow. You don't
have to take it too literally.