Nonparametric tests use the median instead of the mean to calculate differences between groups. These tests have parametric analogues that use the mean. Some common nonparametric tests include the Wilcoxon test, Mann-Whitney U test, and Kruskal-Wallis test, which correspond to the t-test, independent samples t-test, and one-way ANOVA respectively. Z-tests are also nonparametric and used to compare proportions between samples and populations or between two samples.