这个分布的获得方法如下。让x和y是两个大小为m和n的随机独立样本，那么Wilcoxon秩和统计是y[j]不大于x[i]的所有对（x[i], y[j]）的数量。这个统计量的取值在0和m*n之间，其平均值和方差分别为m*n/2和m*n*（m+n+1）/12。如果前三个参数中的任何一个是向量，则使用循环规则对这三个参数的所有组合进行计算，直至最长的向量的长度。

Density, distribution function, quantile function and random generation for the distribution of the Wilcoxon rank sum statistic obtained from samples with size m and n, respectively.

vector of quantiles.

vector of probabilities.

number of observations. If length(nn) > 1, the length is taken to be the number required.

numbers of observations in the first and second sample, respectively. Can be vectors of positive integers.

logical; if TRUE, probabilities p are given as log(p).

logical; if TRUE (default), probabilities are P[X ≤ x], otherwise, P[X > x].