[R] Query about wilcox.test() P-value

[Peter Ehlers]

On 2010-07-14 3:53, Govind Chandra wrote:
> Hi,
>
> I find that the p-value printed out by wilcox.test() and the p-value
> stored in the p.value attribute in the object returned by
> wilcox.test() are not the same. There seems to be a lower limit of
> 2.2e-16 for the printed value although it does say that it is less
> than that. What I want to know is the reason for the lower limit in
> the printed value of p-value and also whether I am doing the right
> thing by picking up the p-value from the p.value attribute of the
> returned object. An example R session is pasted below (although the
> test is probably not the right one for the kind of data).
>
>>   x<- rnorm(500, mean = 30, sd = 3);
>>   y<- rnorm(500, mean = 8000, sd = 6);
>>   wilcox.test(x, y, alternative = "l");
>
>          Wilcoxon rank sum test with continuity correction
>
> data:  x and y
> W = 0, p-value<  2.2e-16
> alternative hypothesis: true location shift is less than 0
>
>>   wt<- wilcox.test(x, y, alternative = "l");
>>   wt$p.value;
> [1] 2.928121e-165

Are you really interested in P-values smaller than 10^(-16)?
Why? A reported P-value of 3e-165 is certainly not accurate
to 165 decimal places and should perhaps be reported as zero,
as t.test() does.

As to your example: there is no sense at all in doing a
test on such data (other than to satisfy some hypothetical
fanatical journal editor).

   -Peter Ehlers


>
> My version for R is 2.11.1 (2010-05-31) running on x86_64 GNU/Linux
> (RHEL).
>
> Thanks in advance for any help with this.
>
> Govind
>
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