什么叫not meet distributional assumption？

问题如下：

Use of a nonparametric test is most appropriate when the

选项：

A.population variance is unknown

B.data are given in ranks

C.distribution is symmetrical

解释：

Nonparametric procedures are primarily used in three situations: when the data are given in ranks, when the data do not meet distributional assumptions, or when the hypothesis being address eddoes not concern a parameter.