Statistics Corner: Wilcoxon–Mann–Whitney Test

JOURNAL TITLE: Journal of Postgraduate Medicine, Education and Research

Author
ISSN
2277-8969
DOI
10.5005/jp-journals-10028-1613
Volume
56
Issue
4
Publishing Year
2022
Pages
3
Article keywords
Brunner–Munzel test, Data interpretation, Mann–Whitney test, Nonparametric test, Two unpaired groups, Wilcoxon rank sum test

Abstract

Wilcoxon–Mann–Whitney (WMW) test is a nonparametric counterpart of the t-test for comparing two unpaired groups. Traditional teaching and many books recommend applying WMW when: (1) continuous outcome variables violate assumptions and (2) data are ordinal. Standard recommendations about the applicability of WMW are not correct. Many health researchers also believe that WMW compares medians between groups; the reporting measure, however, is contextual—it depends on factors such as distribution type, sample size, and heteroscedasticity. A researcher comparing outcomes from two groups found that continuous dependent variables (DVs) do not fulfill the normality and homogeneity of variance assumptions. An initial literature search indicates that nonparametric methods are better for analyzing data. There are, however, a few vital questions concerning analyzing data with WMW: • Does the test make any assumptions? • What it compares—median or mean rank? • What is the null and alternate hypothesis? • What to report and how to interpret results?

© 2019 Jaypee Brothers Medical Publishers (P) LTD.   |   All Rights Reserved