[R] Wilcoxon Rank Sum Test.

[Nordlund, Dan (DSHS/RDA)]

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Marcus Vinicius
> Sent: Thursday, June 28, 2007 1:32 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Wilcoxon Rank Sum Test.
> 
> Dear,
> 
> I'm using R software to evaluate Wilcoxon Rank Sum Test and 
> I' getting one
> Warning message as this:
> 
> > C1dea_com
>  [1] 1.000 0.345 0.200 0.208 0.508 0.480 0.545 0.563 0.451 
> 0.683 0.380 0.913
> 1.000 0.506
> > C1dea_sem
>  [1] 1.000 0.665 0.284 0.394 0.509 0.721 0.545 0.898 0.744 
> 0.683 0.382 0.913
> 1.000 0.970
> 
> 
> > wilcox.test(C1dea_sem,C1dea_com, paired = TRUE, alternative 
> = "two.sided")
> 
>         Wilcoxon signed rank test with continuity correction
> 
> data:  C1dea_sem and C1dea_com
> V = 45, p-value = 0.009152
> alternative hypothesis: true mu is not equal to 0
> 
> Warning message:
> Cannot compute exact p-value with zeroes in: 
> wilcox.test.default(C1dea_sem,
> C1dea_com, paired = TRUE, alternative = "two.sided")
> 
> What is happening?
> 
> Best Regards,
> 
> Marcus Vinicius
> 

Marcus,

It means that you have one or more pairs of observations (5 in your case) where the difference is 0.  The wilcox.test can only compute an approximate p-value under these circumstances.

Hope this is helpful,

Dan

Daniel J. Nordlund
Research and Data Analysis
Washington State Department of Social and Health Services
Olympia, WA  98504-5204