Chapter 13. The Wilcoxon Rank-Sum Test
13.1 Introduction 263
13.2 Synopsis 263
13.3 Examples 265
13.4 Details & Notes 270
The Wilcoxon rank-sum test is a non-parametric analog of the two-sample t-test (Chapter 5). It is based on ranks of the data, and is used to compare location parameters, such as the mean or median, between two independent populations without the assumption of normally distributed data.
Although the Wilcoxon rank-sum test was developed for use with continuous numeric data, the test is also applied to the analysis of ordered categorical data. In clinical data analysis, this test is often useful, for example, for comparing patient global assessments or disease severity ratings between two treatment groups.