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

From: David Winsemius <dwinsemius_at_comcast.net>
Date: Thu, 24 Jun 2010 13:40:55 -0400

>
> 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."

```--
David.

>
>> On Wed, Jun 23, 2010 at 10:27 PM, Atte Tenkanen <attenka_at_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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