Effect size of the Mann-Whitney-Wilcoxon-test in SPSS

// Effect size of the Mann-Whitney-Wilcoxon-test in SPSS //

After conducting the Mann-Whitney-Wilcoxon-test in SPSS with the result of a difference between the two group medians, an effect size is needed, in order to quantify/classify the magnitude of this effect.
For the Mann-Whitney-Wilcoxon-test, r is a suitable effect size.

Unfortunately, SPSS does not provide a calculation within its output. A manual calculation is necessary and consists of a simple division of the standardized test statistic z by the square root of the number of observations.

After the effect size calculation a classification of the magnitude is recommended. Use similar studies or research-field specific thresholds.

For the social and behavioral sciences, the paper Cohen, J. (1992), A Power Primer, p. 157 is usually cited:

📚 Cohen, J. (1992): Quantitative methods in psychology: A power primer. Psychological bulletin, pp. 155-159.


⏰ Timestamps:
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0:00 Introduction
0:10 Calculation of the effect size r
0:54 Classification of the effect size r


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