From: Atte Tenkanen <attenka_at_utu.fi>

Date: Fri, 25 Jun 2010 09:22:56 +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 Fri 25 Jun 2010 - 07:19:15 GMT

Date: Fri, 25 Jun 2010 09:22:56 +0300

BTW. If there is not so weak test that would be suitable for my purpose (because of the ties and the shape of the data), could I proceed this way:

It is also worth of comparing different samples taken from the data. Since the mean and sd of the data are available, could I approximate p-values using z- or t-test, just to compare several different samples?

Atte

> 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 n¡Ö250-300
**> >
**>
**> 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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