The Mann Whitney test is
the nonparametric version of the
two sample
ttest; it compares the means
of two samples. It tests the hypothesis:
H_{0} the samples are drawn from
populations with equal means
H_{1} the mean of the population
of sample 'm' is [less than/greater than/not
equal to] that of sample 'n'
It is similar in concept to the Wilcoxon
Signed Rank Test and is also
known as the Wilcoxon Rank Sum Test. The
test involves:
 sort the combined results into size
order and allocate ranks
 find the sum 'w' of the ranks of the
sample with the smallest number of values
(if they both have the same number of
values, select either). This is one of
the critical values.
For small data sets the critical value
is found from published tables. Because
the sum of the ranks must be an integer,
it is not usually possible to find the exact
critical value:
m 
number of values in the sample with
the least number of values 
n 
number of values in the other sample 
μ _{1} 
the mean of the sample containing
the smallest number of values 
