The 2-Sample Mann-Whitney (or two-sample Wilcoxon rank sum test) is a nonparametric test of the equality of two population medians. It also calculates the corresponding point estimate and confidence interval. Its parametric counterpart is the Paired t test. This procedure assumes that the samples are randomly and independently drawn from two populations that have the same shape.
| Test | Null Hypothesis, H0 | Alternate Hypothesis, H1 |
| One-tailed | m1 = m2 | m1 < m2 or m1 > m2 |
| Two-tailed | m1 = m2 | m1 <> m2 |
where m1 and m2 are population medians
At the Excel Menu (For Excel 2007, go to Add-ins first)
Choose ProcessMA > Statistics > Nonparametric > 2-Sample Mann-Whitney
In Variable 1, select the column containing the data for the first sample (Numeric)
In Variable 2, select the column containing the data for the second sample (Numeric)
In Alternate, select the appropriate alternate test
Click OK
The number of printers sold by two teams of salespeople was recorded for the previous month. You want to find out if there is a difference in the population median of sales between the teams.
Open data worksheet by choosing ProcessMA > Tools > Data
Choose ProcessMA > Statistics > Nonparametric > 2-Sample Mann-Whitney
In Variable 1, select BO - Sales A
In Variable 2, select BP - Sales B
Click OK
The median for both sales team A and B are 51 and 52 respectively. For a desired a = 0.05, since p = 0.7995 > a, we fail to reject H0. Therefore, we conclude that there is no significant evidence that the sales of the two teams are the different.
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