Exact and Approximate Distributions for the Wilcoxon Statistic with Ties
The exact distribution of the Wilcoxon statistic was calculated for five specific cases where both samples contained five observations, some of which were tied. The five specific cases were obtained from an actual experiment. The exact distributions for each of these five cases are compared with some approximations at points near 1, 5, and 10 per cent. Critical values and associated probabilities have been calculated for the exact and approximate distributions
Lehman, SY. (1961). Exact and Approximate Distributions for the Wilcoxon Statistic with Ties. Journal of the American Statistical Association, 56(294), 293-298.