Mann Whitney U Test

The Mann Whitney U Test is a non-parametric test for comparing the means of two (independent) groups.

Assumptions
The assumptions of this test are as follows:
 * 1) Data must be at least at the ordinal level.
 * 2) Population numbers must be equal across groups.

Parametric Equivalent
The parametric equivalent of this test is the Independent T-Test.

Example
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In SPSS
Click Analyze > Nonparametric Tests > Legacy Dialogs > 2 Related Samples...

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Writing up
From this data, it can be concluded that there is a statistically significant difference between the exercise and diet treatment group's median cholesterol concentration at the end of both treatments (U = 110, p = 0.014). It can be further concluded that the exercise treatment elicited statistically significant lower cholesterol concentrations than the dietary group (p = 0.014).