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.
On Wed, Jun 23, 2010 at 10:27 PM, Atte Tenkanen <atte...@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.m...@ugent.be
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