# MASH : Maths and Stats Help

### Mann-Whitney U-Test, using SPSS

#### Introduction

A Mann-Whitney U-Test compares the means between two unrelated groups, such as comparing the difference in heights between Cats and Yorkshire Terriers.

It is considered a non-parametric test and therefore is suitable for non-parametric data. To check if your data is parametric, please check our dedicated guide: Parametric or Not Guide (PDF)

If your data is parametric you should consider using an Independent-Samples t-Test

#### Test Procedure

1. Click Analyze > Nonparametric Tests > Legacy Dialogs > 2 Independent Samples

2. Within the Two-Independent-Samples box, select the test variable or dependent variable you are analysing and move it to the ‘Test Variable List’ box. Then move your independent/grouping variable into the “Grouping Variable” box.

3. Select Define Groups and specify the groups you are going test. In this example I am testing Cats which are labelled 1 against Yorkshire Terroirs which are labelled 2, therefore my groups are numbered 1 and 2.

4. Select Options and click Descriptive Statistics

5. Click "Continue" then "OK"

#### Results

SPSS will generate three tables, for this test we need two, the Descriptive Statistics and the Test Statistics:

##### Descriptive Statistics

This table shows a selection of descriptive statistics: the sample size of each group (N), the mean of each group (Mean), and the standard deviation of each group (Std. Deviation), best practice is to report them all.

##### Test Statistics

This table shows the specific test results including the Mann-Whitney U statistic (U), the Wilcoxon Statistic (W), the z-score (Z) and two different p-values / significance values. For the vast majority of applications, you will need to report the Asymp. Sig (2-tailed) version of the p-value.

## Reporting the Results in APA Formatting

Heights of Cats and Yorkshire Terriers were compared. On average the Yorkshire Terriers (Mdn = 29.27) were taller than the Cats (Mdn = 28.00) however a Mann-Whitney U test indicated that this difference was not statistically significant, (NCats=11, NTerriers=11) = 35.5, z = -1.67, = .094.