Nonparametric Two‐Sample Tests

This chapter looks at the equivalent of the t-test for non normally distributed data. The Mann–Whitney test is for unpaired data drawn from a population of observations in which the variable is not normally distributed. It assumes that there are two independent samples. It is the nonparametric equivalent of the unpaired t-test. The Wilcoxon signed-rank test is used for paired samples. It is the nonparametric equivalent of the paired t-test. The Wilcoxon Sign test is for paired samples where there is a binary outcome. The Wilcoxon sign test was most famously used in 1950s ads for washing powder. Statistical package for the social sciences (SPSS) provides useful and easy-to-interpret output for the nonparametric tests. The chapter presents the SPSS output for Mann–Whitney analysis, Wilcoxon sign test and Wilcoxon signed-rank test.