Mann Whitney test.

[John Darrington]

Jason and I were discussing the Mann-Whitney test on IRC last week,
which inspired me to look up the literature on it.

One interesting paper I found is Bergmann, Ludbrook and Spooren,
"Different Outcomes of the Wilcoxon-Mann-Whitney Test from Different
Statistics Packages", The American Statistician 54(1), 2000, pp 72-77

From their conclusion:
 "... statistics packages provide very inadequate documentation in
 their manuals... As a consequence, the results can be dangerously
 misleading. 
 Investigators cannot rely on the popular, general purpose,
 microcomputer statistics programs ... to provide an accurate outcome
 of the WMW test."

Earlier in the paper, they say the following about spss:

 "The second `exact' p value presumably results from Dineen and
 Blakeksley (1973) algorithm, even though this is invalid when there
 are ties."


We ought to think very carefully therefore when implementing this in
PSPP, and I rather agree with the authors that most stats programs'
documentation is non-existent.

J'


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