# Re: [R] Wilcoxon signed rank test and its requirements

From: Atte Tenkanen <attenka_at_utu.fi>
Date: Thu, 24 Jun 2010 05:16:19 +0300

PS.

>
> how do you test that the data is symmetric enough?
> If it is not, is it ok to use some data transformation?
>
> when it is said:
>
> "The Wilcoxon signed rank test does not assume that the data are
> sampled from a Gaussian distribution. However it does assume that the

> data are distributed symmetrically around the median. If the
> distribution is asymmetrical, the P value will not tell you much about
> whether the median is different than the hypothetical value."
>
> > On Wed, Jun 23, 2010 at 10:27 PM, Atte Tenkanen <attenka@utu.fi> wrote:
> > > Hi all,
> > >
> > > I have a distribution, and take a sample of it. Then I compare
> that
> > sample with the mean of the population like here in "Wilcoxon signed

>
> > rank test with continuity correction":
> > >
> > >> wilcox.test(Sample,mu=mean(All), alt="two.sided")
> > >
> > >        Wilcoxon signed rank test with continuity correction
> > >
> > > data:  AlphaNoteOnsetDists
> > > V = 63855, p-value = 0.0002093
> > > alternative hypothesis: true location is not equal to 0.4115136
> > >
> > >> wilcox.test(Sample,mu=mean(All), alt = "greater")
> > >
> > >        Wilcoxon signed rank test with continuity correction
> > >
> > > data:  AlphaNoteOnsetDists
> > > V = 63855, p-value = 0.0001047
> > > alternative hypothesis: true location is greater than 0.4115136
> > >
> > > What assumptions are needed for the population?
> >
> > wikipedia says:
> > "The Wilcoxon signed-rank test is a _non-parametric_ statistical
> > hypothesis test for... "
> > it also talks about the assumptions.
> >
> > > What can we say according these results?
> > > p-value for the "less" is 0.999.
> >
> > That the p-value for less and greater seem to sum up to one, and that

> > the p-value of greater is half of that for two-sided. You shouldn't
> > ask what we can say. You should ask yourself "What was the question
> > and is this test giving me an answer on that question?"
> >
> > Cheers
> > Joris
> >
> > --
> > Joris Meys
> > Statistical consultant
> >
> > Ghent University
> > Faculty of Bioscience Engineering
> > Department of Applied mathematics, biometrics and process control
> >
> > tel : +32 9 264 59 87
> > Joris.Meys_at_Ugent.be
> > -------------------------------
> > Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php

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