- Nonparametric version of the independent samples t-test
- Evaluates difference between 2 treatment conditions or populations
- Mnemonic; the U stands for unrelated (i.e. independent)
- Do NOT use means; just higher/lower scores = ranks; Tests for differences between groups. NOT DIFFERENCES BETWEEN MEANS

- Nonparametric version of the independent samples t-test - Evaluates difference between 2 treatment conditions ________

Mann-Whitney U test

- Scores in one sample generally larger than the other
- If combined, scores from each sample should be concentrated at either ends

Mann Whitney U data if there is a real difference

- Then large and small scores will be mixed evenly in the two samples

Mann Whitney U data if there is NO real difference

- Distribution free, not of any fixed parameter, parameter flexible- assumption free
- The data to be analysed is not required to fit a normal distribution

Ua = NaNb + Na (Na + 1) divided by 2 MINUS the sum of Ra (rank sum)

- U = idea that 2 samples are 2 teams competing in a race; each person in group A gets a point if ranked ahead of someone from group B (UA) vice versa for group B (UB) 

- Na = ppts in sample A; Nb = ppts in sample B

- The Mann-Whitney U value is the smaller of the two values
- The calculated U must be equal to or smaller than critical value to be significant
- For the largest possible difference, one sample would have no points, therefore U = 0 (lowest)

that the sample with the lowest ranks has very few points, and indicates a significant difference

that the samples are very similar to each other (as the ranks are now intermixing and the points are assigned more evenly between the samples)

A Mann-Whitney U test was used to compare the ranks of people with low blood alcohol level (n =12), with those with high blood alcohol level (n = 13). The analysis did not indicate a difference in the reaction time scores of the two groups: U = 45, p &gt; .05, two-tailed, with the sum of the ranks totalling 123 for the low blood alcohol group, and 202 for those with high blood alcohol.

Click Analyse, select Nonparametric Tests, Legacy Dialog and click on Two Independent Samples.

Study with Quizlet and memorize flashcards containing terms like What is the Mann-Whitney U test?, Mann Whitney U data if there is a real difference, Mann Whitney U data if there is NO real difference and more.

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