The nonparametric Wilcoxon-Mann-Whitney test is commonly used by practitioners for detecting differences in location (mean, median) between two samples. Earlier work (e.g., Zimmerman (1987), Feltovich (2003)) has shown that this test has a number of disadvantages, most of which are ameliorated by the robust rank-order test (Fligner and Pollicello (1981)). Use of the robust rank-order test has been limited, partly because exact critical values have heretofore been available for only a small number of sample-size values, and not for all of the commonly-used levels of significance. This note expands what is known about the distribution of the robust rank-order test statistic; critical values are given for more sample sizes and for more levels of significance.
Feltovich, Nick (2005), "Critical values for the robust rank-order test," Communications in Statistics - Simulation and Computation 34 (3), pp. 525-547. DOI: 10.1081/SAC-200068395.