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

Govind Chandra govind.chandra at bbsrc.ac.uk
Wed Jul 14 11:53:58 CEST 2010


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

My version for R is 2.11.1 (2010-05-31) running on x86_64 GNU/Linux

Thanks in advance for any help with this.


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