> -----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