From: Atte Tenkanen <attenka_at_utu.fi>

Date: Thu, 24 Jun 2010 22:36:18 +0300

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu 24 Jun 2010 - 20:17:44 GMT

Date: Thu, 24 Jun 2010 22:36:18 +0300

> On Jun 23, 2010, at 9:58 PM, Atte Tenkanen wrote:

*>
**> > Thanks. What I have had to ask is that
**> >
**> > 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."
**>
**> You are being misled. Simply finding a statement on a statistics
**> software website, even one as reputable as Graphpad (???), does not
**> mean that it is necessarily true. My understanding (confirmed
**> reviewing "Nonparametric statistical methods for complete and censored
**>
**> data" by M. M. Desu, Damaraju Raghavarao, is that the Wilcoxon signed-
**>
**> rank test does not require that the underlying distributions be
**> symmetric. The above quotation is highly inaccurate.
**>
**> --
**> David.
*

Thanks. Unfortunately, I can't follow the reference at all, but I read this in that way that I can be carefree as far as the underlying distribution is concerned?

Is there any other authoritative reference where that is just stated in a way "test does not require that the underlying distributions be symmetric or normal".

Atte

*> >
*

> >> 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
**> >
**> > ______________________________________________
**> > R-help_at_r-project.org mailing list
**> > https://stat.ethz.ch/mailman/listinfo/r-help
**> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
**> > and provide commented, minimal, self-contained, reproducible code.
*

>

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