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The Stata Journal
Volume 12 Number 2: pp. 182-190

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What hypotheses do “nonparametric” two-group tests actually test?

Ronán M. Conroy
Royal College of Surgeons in Ireland
Dublin, Ireland
Abstract.  In this article, I discuss measures of effect size for two-group comparisons where data are not appropriately analyzed by least-squares methods. The Mann–Whitney test calculates a statistic that is a very useful measure of effect size, particularly suited to situations in which differences are measured on scales that either are ordinal or use arbitrary scale units. Both the difference in medians and the median difference between groups are also useful measures of effect size.
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View all articles with these keywords: ranksum, Wilcoxon rank-sum test, Mann–Whitney statistic, Hodges–Lehman median shift, effect size, qreg

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