[R] Interpreting the result of a Wilcoxon (Mann-Whitney U) test

Filipe Correia fcorreia at gmail.com
Wed Oct 2 22:31:39 CEST 2013


Thank you for your answer Charles.

On Wed, Oct 2, 2013 at 6:50 PM, Charles Determan Jr <deter088 at umn.edu> wrote:
> When you chose a different alternative argument you are asking a different
> null hypothesis.

I was considering the null hypothesis was expressed through the "mu"
argument. I think I got this idea from R's docs where it states that
"the null hypothesis is that the distributions of x and y differ by a
location shift of mu" (when "a" and "b" are both given and "paired" is
FALSE).  Does the "alternative" argument also influence the null
hypothesis in some way that I may be missing?

My understanding was that by using the "alternative" argument one
would be expressing the alternative hypothesis only.

> You are looking at a two-tailed, lesser than, and greater
> than hypotheses.  Which one you chose is dependent upon your initial
> question.  Are you asking generically if your two populations (a and b) are
> different?  Are you asking if a > b or a < b?

Well, I guess I'm asking all of the three... :)
Not with the intent to choose which one fits, but because I was
experimenting and making sure I really understand how I could use R's
implementation of the Wilcoxon test. But I'm intrigued that the answer
to "are a and b different" is not consistent with "is a less than
b"... Maybe my understanding of the function's arguments and return
values is not correct? :\


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