Rank sum test (for two unpaired Samples)
from Wilcoxon

H0: The two distributions of X und Y are equal
H1: The two distributions of X und Y are different

Remark:
The distributions are F_{X} and F_{Y}, where F_{X} is
the distribution of X_{1}, ..., X_{m} and F_{Y}
is the distribution of Y_{1}, ..., Y_{n}.