The test is used to determine if one of two groups of data tends to have more widely dispersed values than the other.
In other words, the test determines whether one of the two groups tends to move, sometimes to the right, sometimes to the left, but away from the center (of the ordinal scale).
The test was published in 1960 by Sidney Siegel and John Wilder Tukey in the Journal of the American Statistical Association, in the article "A Nonparametric Sum of Ranks Procedure for Relative Spread in Unpaired Samples."
If one of the two groups is more dispersed its ranks will be lower, as extreme values receive lower ranks, while the other group will receive more of the high scores assigned to the center.
From the rank sums the U statistics are calculated by subtracting off the minimum possible score, n(n + 1)/2 for each group:[1] According to