To test whether there is a significant shift in values of a variable
for a collection of observations after some event.
Nonparametric tests appropriate when the distribution of variable
in is not normally distributed. It might be
right skewed or bi-modal, for example.
For small samples (10's) traditional tests for significance that
assume a normally distributed sample would not be appropriate.
Wilcoxon Mann-Whitney tests two populations with the null hypothesis
being the two populations are similar and the alternative hypothesis
being the two populations differ with respect to the median. This
is the nonparametric equivalent of the two sample t-test.
Wilcoxon Signed Rank test also compares population medians but uses
matched pairs, so it is suitable for analyses of variable before and
after an event, for the same entity.
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The PDF version is a formatted comprehensive draft book (with over 800 pages).
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