Mann-Whitney U Test using SPSS

Objectives

The Mann-Whitney U Test is used to compare differences between two independent groups when the dependent variable is either (a) ordinal or (b) interval but not normally distributed. It is the nonparametric alternative to the independent t-test.

Assumptions

Random samples from populations
The dependent variable is either ordinal, interval or ratio (see our Types of Variable guide for help)
Samples do NOT need to be normally distributed

Example

A random sample of overweight, male individuals were recruited to a study to investigate whether an exercise or weight loss intervention would be more effective at lowering blood cholesterol levels in overweight men. To this end, researchers randomly split their sample group into two equally-sized, smaller groups; one group underwent an exercise training programme and the other group undertook a calorie-controlled, weight-loss diet. In order to determine which treatment programme (exercise or diet) was more effective, cholesterol concentrations were compared between the two groups at the end of the treatment programmes.

Setup in SPSS

In SPSS we separated the groups for analysis by creating a grouping variable called "Group" and gave the exercise group a value of "1" and the diet group a value of "2". Cholesterol concentrations were entered under the variable name "Cholesterol". To correctly setup your data in SPSS to run a Mann-Whitney U Test please read our Entering Data in SPSS tutorial.

Testing Assumptions

Cholesterol concentration is measured on a continuous measurement scale (specifically, a ratio scale) and thus meets the variable requirements for this test. However, having tested the normality of the cholesterol data in the two treatments we have been able to conclude that one of the groups is not normally distributed. Therefore, we cannot use an Independent T-Test and have decided to run a Mann-Whitney U Test. The procedure for checking the normality of your data is provided in our Testing for Normality guide.

Test Procedure in SPSS

Click Analyze > Nonparametric Tests > Legacy Dialogs > 2 Independent Samples... on the top menu as shown below:

Published with written permission from IBM SPSS Inc.
You will be presented with the following screen:

Published with written permission from IBM SPSS Inc.
Move the dependent variable "Cholesterol" to the "Test Variable List:" box and the independent variable "Group" to the "Grouping Variable:" box by using the button or by dragging-and-dropping the variables into the boxes.

Published with written permission from IBM SPSS Inc.
Make sure that the "Mann-Whitney U" checkbox is ticked in the "Test Type" area and the "Grouping Variable:" box is highlighted in yellow (as seen above). If not highlighted in yellow, simply click your cursor in the box.
Click on the button. The button will not be clickable if you have not highlighted the "Grouping Variable:" box as instructed in Point 4. You will be presented with the following screen:

Published with written permission from IBM SPSS Inc.