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

From: David Winsemius <dwinsemius_at_comcast.net>
Date: Thu, 24 Jun 2010 23:52:45 -0400

On Jun 24, 2010, at 6:58 PM, Atte Tenkanen wrote:

> Is there anything for me?
>
> There is a lot of data, n=2418, but there are also a lot of ties.
> My sample n250-300
>

I do not understand why there should be so many ties. You have not described the measurement process or units. ( ... although you offer a glipmse without much background later.)

> i would like to test, whether the mean of the sample differ
> significantly from the population mean.

Why? What is the purpose of this investigation? Why should the mean of a sample be that important?

>
> The histogram of the population looks like in attached histogram,
> what test should I use? No choices?
>
> This distribution comes from a musical piece and the values are
> 'tonal distances'.
>
> http://users.utu.fi/attenka/Hist.png

That picture does not offer much insidght into the features of that measurement. It appears to have much more structure than I would expect for a sample from a smooth unimodal underlying population.

-- 
David.


>
> Atte
>
>> On 06/24/2010 12:40 PM, David Winsemius wrote:
>>>
>>> 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.
>>>
>>
>> To add to what David and others have said, look at the kernel that
>> the
>>
>> U-statistic associated with the WSR test uses: the indicator (0/1) of
>> xi
>> + xj > 0. So WSR tests H0:p=0.5 where p = the probability that the
>> average of a randomly chosen pair of values is positive. [If there
>> are
>> ties this probably needs to be worded as P[xi + xj > 0] = P[xi + xj <
>>
>> 0], i neq j.
>>
>> Frank
>>
>> --
>> Frank E Harrell Jr Professor and Chairman School of Medicine
>> Department of Biostatistics Vanderbilt
>> University
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