[R] minmun p-value for wilcox.test and correlation

Peter Dalgaard p.dalgaard at biostat.ku.dk
Wed May 20 22:54:47 CEST 2009


charles78 wrote:
> I have a stupid question on how to get the real p-values for wilcox.test and
> correlation.  the minmun can be reached is 2.2E-16 using the R version
> 2.6.2.  I do not think it is the R version causing this but other issues.
> 
> Any help is highly appreciated.
> 
> Charles

It's mainly a printing issue (and to some extent historical, because 
small p-values in the upper tail were generated by subtraction and could 
underflow if < 2.2e-16):

 > wilcox.test(1:100,101:200)

	Wilcoxon rank sum test with continuity correction

data:  1:100 and 101:200
W = 0, p-value < 2.2e-16
alternative hypothesis: true location shift is not equal to 0

 > wilcox.test(1:100,101:200)$p.value
[1] 2.562144e-34

-- 
    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
  (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907



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