[R] interprete wilcox.test results

[Torsten Hothorn]

On Wed, 8 Nov 2006, Baoqiang Cao wrote:

> Dear All,
>
> I am using wilcox.test to test two samples, data_a and data_b, earch sample has 3 replicates, suppose data_a and data_b are 20*3 matrix. Then I used the following to test the null hypothesis (they are from same distribution.):
>
> wilcox.test(x=data_a, y=data_b, alternative="g")
> I got pvalue = 1.90806170863311e-09.
>
> When I switched data_a and data_b by doing the following:
> wilcox.test(x=data_b, y=data_a, alternative="g")
> I got pvalue = 0.999999998111886.
>
> I suppose because I am test them with same null hypothesis,

no, you don't:

H_0: mean(data_a) <= mean(data_b) (alternative = "greater")

is different from

H_0: mean(data_b) <= mean(data_a)

check

wilcox.test(x=data_a, y=data_b, alternative="l")

Torsten

> I should get same result (pvalue). Since I am not a savy statistician at 
> all, maybe that is a wrong assumption, could anyone please point out the 
> right interpretion please?
>
> Thanks!
>
> Best,
> Baoqiang
>
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