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≈250-300
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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