Example of 2-sample Mann-Whitney test
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Samples were drawn from two populations and diastolic blood pressure was measured. You will want to determine if there is evidence of a difference in the population locations without assuming a parametric model for the distributions. Therefore, you choose to test the equality of population medians using the Mann-Whitney test with a = 0.05 rather than using a two-sample t-test, which tests the equality of population means.

1    Open the worksheet EXH_STAT.MTW.

2    Choose Stat > Nonparametrics > Mann-Whitney.

3    In First Sample, enter DBP1. In Second Sample, enter DBP2. Click OK.

Session window output

Mann-Whitney Test and CI: DBP1, DBP2

 

 

      N  Median

DBP1  8   69.50

DBP2  9   78.00

 

 

Point estimate for η1 - η2 is -7.50

95.1 Percent CI for η1 - η2 is (-18.00,4.00)

W = 60.0

Test of η1 = η2 vs η1 ≠ η2 is significant at 0.2685

The test is significant at 0.2679 (adjusted for ties)

Interpreting the results

Minitab calculates the sample medians of the ordered data as 69.5 and 78. The 95.1% confidence interval for the difference in population medians (ETA1-ETA2) is [-18 to 4]. The test statistic W = 60 has a p-value of 0.2685 or 0.2679 when adjusted for ties. Since the p-value is not less than the chosen a level of 0.05, you conclude that there is insufficient evidence to reject H0. Therefore, the data does not support the hypothesis that there is a difference between the population medians.