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Mann-Whitney Test for Two Samples Help

The Mann-Whitney test is used when you want to determine if the results from one population or larger than the results from another population. For example, you might want to do if a group of patients treated with a new vaccine did not get ill as often as a group of untreated patients. The example below shows how to do this test using the SPC for Excel software. The median is once again used in this test since it is a better measure of the central tendency of the data than the average for non-normal data.

Example (from Statistics and Data Analysis, by Ajit Tamhane and Dorothy Dunlop, Prentice-Hall, 2000)

The data below is the failure time for 18 capacitors. Eight had been tested under normal operating conditions; ten had been tested under thermally stress conditions.

Normal: 5.2, 8.5, 9.8, 12.3, 17.1, 17.9, 23.7, 29.8

Stress: 1.1, 2.3, 3.2, 6.3, 7, 7.2, 9.1, 15.2, 18.3, 21.1

We want to use these results to see if there is a difference between the two groups.

Mann-Whitney Test for Two Samples Output

The output from the Mann-Whitney test is shown below.

The conclusions, based on value of p and alpha, are given. Reject the null hypothesis if p value < alpha.


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