subset of tied elements, irrespective of the group
the tied element belongs to. If the number of ties
is relatively small, Mann–Whitney:Wilcoxon’s
probabilities are good approximations. In cases
with no or relatively few ties, good approximations
can also be obtained by a normal distribution
without any corrections. As discussed in
almost every textbook on statistics, this is based
on the fact that if either group size is at least 20,
the rank sums follow an almost perfect normal
distribution with mean n1(n1n21):2 for the
smaller group and n2(n1n21):2 for the larger