Rank Randomization Tests (3 of 4)
The two-tailed probability is therefore: 8/70 = 0.114.
Notice that this probability value is slightly higher than the 0.0857
obtained with the randomization test for these same data
This rank randomization test for differences between two groups has several
names. It is most often called the Mann-Whitney U test or the Wilcoxon
Rank Sum test. In practice, tables are available to look up the probability
value based on the sum of the ranks of the group with the lower mean
rank. For these data, the sum of the ranks is: 2+6+1+3 =12. A table
would show that for a one-tailed test to be significant at the
, the sum of the ranks must be ≤
11. Since 12>11, the test is not significant at this level. This agrees
with the calculated probability value of 0.057. The purpose of this
section is to present the concepts rather than the details of the computations.
Therefore, if you wish to actually perform this test, you should find
a textbook with appropriate tables.
An online table and further discussion can be