[Scipy-tickets] [SciPy] #1593: Mann-Whitney statistic returns incorrect results
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Sun Feb 5 19:21:57 CST 2012
#1593: Mann-Whitney statistic returns incorrect results
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Reporter: rgommers | Owner: somebody
Type: defect | Status: new
Priority: normal | Milestone: Unscheduled
Component: scipy.stats | Version: 0.10.0
Keywords: mannwhitneyu |
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Comment(by warren.weckesser):
The value 102 in the above example is correct. See the description of the
calculation in the wikipedia article:
http://en.wikipedia.org/wiki/Mann%E2%80%93Whitney_U#Calculations
In particular, "The smaller value of U1 and U2 is the one used when
consulting significance tables." An elementary statistics text that I
happen to have handy (Probability and Statistics for Engineers (7th ed),
by Richard A. Johnson) says the same thing: "...the statistic U, which
always equals the smaller of the two." mannwhitneyu() is returning the
smaller value.
Because of the symmetry of the calculation, either U1 or U2 can be used to
compute Z (since U1 + U2 = n1*n2).
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Ticket URL: <http://projects.scipy.org/scipy/ticket/1593#comment:2>
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