# [R] about a p-value < 2.2e-16

Bogdan Tanasa t@n@@@ @end|ng |rom gm@||@com
Fri Mar 19 06:15:45 CET 2021

```Dear Spencer, thank you very much for your prompt email and help. When
using :

> wilcox.test(rnorm(100), rnorm(100, 2), exact=TRUE)
W = 698, p-value < 2.2e-16

> wilcox.test(rnorm(100), rnorm(100, 2), exact=FALSE)
W = 1443, p-value < 2.2e-16

and in both cases p-value < 2.2e-16. By "exact" p-value, i have meant the
"precise" p-value ;

If I may ask please, could we write p-value = 0 ?

i have noted a similar conversation on stackexchange, although the answer
is not very clear (to me).

https://stats.stackexchange.com/questions/78839/how-should-tiny-p-values-be-reported-and-why-does-r-put-a-minimum-on-2-22e-1

thanks again,

bogdan

On Thu, Mar 18, 2021 at 10:05 PM Spencer Graves <
spencer.graves using effectivedefense.org> wrote:

>        I would push back on that from two perspectives:
>
>
>              1.  I would study exactly what the journal said very
> carefully.  If they mandated "wilcox.test", that function has an
> argument called "exact".  If that's what they are asking, then using
> that argument gives the exact p-value, e.g.:
>
>
>  > wilcox.test(rnorm(100), rnorm(100, 2), exact=TRUE)
>
>          Wilcoxon rank sum exact test
>
> data:  rnorm(100) and rnorm(100, 2)
> W = 691, p-value < 2.2e-16
>
>
>              2.  If that's NOT what they are asking, then I'm not
> convinced what they are asking makes sense:  There is is no such thing
> as an "exact p value" except to the extent that certain assumptions
> hold, and all models are wrong (but some are useful), as George Box
> famously said years ago.  Truth only exists in mathematics, and
>
>
>        Hope this helps.
>        Spencer Graves
>
>
> 
> https://en.wikipedia.org/wiki/All_models_are_wrong
>
>
> On 2021-3-18 11:12 PM, Bogdan Tanasa wrote:
> >   <
> > Dear all,
> >
> >
> > in R, the wilcox.test() provides "a p-value < 2.2e-16", when we compare
> > sets of 1000 genes expression (in the genomics field).
> >
> > however, the journal asks us to provide the exact p value ...
> >
> > would it be legitimate to write : "p-value = 0" ? thanks a lot,
> >
> > -- bogdan
> >
> >       [[alternative HTML version deleted]]
> >
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