On Wed, 2009-12-02 at 16:52 +0800, Zhijiang Wang wrote:
> Dear All,
> 1. why did the problem happen?
> 2. How to solve it?
>
> --
>
> Best wishes,
> Zhijiang Wang
Well... The algorithm for Mann-whitney test have problem with ties
To solve you can use jitter
a<-1:10
b<-1:10
wilcox.test(a,b)
Wilcoxon rank sum test with continuity correction
data: a and b
W = 50, p-value = 1
alternative hypothesis: true location shift is not equal to 0
Warning message:
In wilcox.test.default(a, b) : cannot compute exact p-value with ties
wilcox.test(a,jitter(b))
Wilcoxon rank sum test
data: a and jitter(b)
W = 49, p-value = 0.9705
alternative hypothesis: true location shift is not equal to 0
look ?jitter for more information
--
Bernardo Rangel Tura, M.D,MPH,Ph.D
National Institute of Cardiology
Brazil
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