Reporting Mann Whitney U Test

Isle of man-Whitney U Examination using SPSS Statistics (cont...)

Enter i into the Group one: box and enter ii into the Group 2: box. Remember that we labelled the Nutrition group as 1 and the Exercise group every bit 2.

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Note: If you have more than than 2 groups in your study (e.1000., three groups: diet, practice and drug groups), but but wanted to compare 2 (eastward.g., the diet and drug groups), y'all could type 1 into the Group 1: box and 3 into the Group two: box (i.e., if you wished to compare the diet with drug grouping).
Click on the button.
If you wish to utilise this procedure to generate some descriptive statistics, click on the button and then tick Descriptive and Quartiles within the –Statistics– area. You volition be presented with the dialogue box below:

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Click on the push, which will bring yous back to the main dialogue box with the Grouping Variable: box now completed, as shown beneath:

Published with written permission from SPSS Statistics, IBM Corporation.
Click on the button. This will generate the output for the Isle of mann-Whitney U examination.

SPSS Statistics

Output and Estimation

If yous have been post-obit this guide from folio i, you will know that the following output and interpretation relates to the Mann-Whitney U examination results when your ii distributions take a dissimilar shape, such that you are comparing mean ranks rather than medians. This is what happens when your information has violated Supposition #4 of the Mann-Whitney U test. The output is also based on the employ of the Legacy Dialogs > 2 Independent Samples procedure in SPSS Statistics. If yous accept used the Nonparametric Tests > Independent Samples procedure in SPSS Statistics or you need to know how to interpret medians because your information has met Assumption #4 of the Mann-Whitney U test, we explain how to exercise this in our enhanced Mann-Whitney U test guide, which yous can access by subscribing to Laerd Statistics.

In the SPSS Statistics output beneath, we show yous how to written report the Mann-Whitney U test using mean ranks. To do this, SPSS Statistics produces three tables of output:

Descriptives

The Descriptive Statistics tabular array looks as follows:

'Descriptive Statistics' table. Mann-Whitney U test with 'N', 'Mean', 'Std. Deviation', 'Minimum', 'Maximum' & 'Percentiles'

Published with written permission from SPSS Statistics, IBM Corporation.

Although we have decided to show y'all how you can go SPSS Statistics to provide descriptive statistics for the Mann-Whitney U test, they are non actually very useful. The reason for this is twofold. Firstly, in order to compare the groups, we need the individual group values, non the amalgamated ones. This table does not provide us with this vital data, so we cannot compare whatsoever possible differences between the exercise and diet groups. Secondly, nosotros chose the Mann-Whitney U test because one of the individual groups (exercise group) was not normally distributed. However, we accept non tested to see if the amalgamation of the two groups results in the larger group being normally distributed. Therefore, we do not know whether to employ the mean and standard deviation or the median and interquartile range (IQR). The IQR is the 25th to 75th percentile. This volition act as a surrogate to the standard deviation we would otherwise written report if the information were ordinarily distributed. For these reasons, we recommend that you lot ignore this table.

Ranks Table

The Ranks table is the start table that provides data regarding the output of the bodily Mann-Whitney U examination. It shows mean rank and sum of ranks for the two groups tested (i.e., the exercise and nutrition groups):

'Ranks' table. Mann-Whitney U test in SPSS. Shows 'N', 'Mean Rank', 'Sum of Ranks', for 'Diet' & 'Exercise' groups

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The table above is very useful because information technology indicates which group can be considered as having the higher cholesterol concentrations, overall; namely, the grouping with the highest mean rank. In this case, the diet group had the highest cholesterol concentrations.

Exam Statistics Tabular array

This tabular array shows us the actual significance value of the test. Specifically, the Test Statistics table provides the test statistic, U statistic, too equally the asymptotic significance (2-tailed) p-value.

'Test Statistics' table. Includes 'Mann-Whitney U', 'Asymp. Sig. (2-tailed)', and others, for 'Cholesterol Concentration'

Published with written permission from SPSS Statistics, IBM Corporation.

From this data, it can be concluded that cholesterol concentration in the diet group was statistically significantly higher than the do grouping (U = 110, p = .014). Depending on the size of your groups, SPSS Statistics volition produce both exact and asymptotic statistical significance levels. Understanding which i to use is explained in our enhanced guide.

In our enhanced Mann-Whitney U test guide, we testify you: (a) how to utilise SPSS Statistics to determine whether your ii distributions take the same shape or a different shape; (b) the ii procedures – Nonparametric Tests > Independent Samples and Legacy Dialogs > 2 Independent Samples – that you lot can apply to deport out a Mann-Whitney U examination; (c) how to use SPSS Statistics to generate medians for the Mann-Whitney U test if your two distributions have the same shape; and (d) how to fully write up the results of the Isle of mann-Whitney U test procedure whether yous are comparison hateful ranks or medians. Nosotros practise this using the Harvard and APA styles. Yous tin can admission our enhanced Mann-Whitney U test guide, every bit well as all of our SPSS Statistics content, past subscribing to Laerd Statistics, or learn more than about our enhanced content in general on our Features: Overview page.