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Categorical Data Analysis Using The SAS® System, 2nd Edition by Gary G. Koch, Charles S. Davis, Maura E. Stokes

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7.2. Wilcoxon-Mann-Whitney Test

The Wilcoxon-Mann-Whitney test is a nonparametric test of the null hypothesis that the distribution of an ordinally scaled response variable is the same in two independently sampled populations. It is sensitive to the alternative hypothesis that there is a location difference between the two populations. This test was first proposed for the case of two samples of equal size by Wilcoxon (1945). Mann and Whitney (1947) introduced an equivalent statistic, and they were the first to consider unequal sample sizes and to furnish tables suitable for use with small samples. (Such tables are found in the appendices of standard statistics textbooks.)

The Wilcoxon-Mann-Whitney test can be used whenever the two-sample t

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