Mann-Whitney Test

Mann-Whitney Test

1. Looking for difference between groups

2. Normally use interval or ratio data

3. We are doing independent groups test

4. Non-parametric

5. mann-whitney test is the non-parametric alternative to the independent t-test

6. looks at the MEDIAN

7. Under the null hypothesis the distributions of both groups are equal, so that the probability of an observation from one population (X) exceeding an observation from the second population (Y) equals the probability of an obsevration from Y exceeding an observation from X

   • there is symmetry between population with respect to probability of random drawing of a larger population

8. z score= the gap between the means

   • z scores are comparable across tests
   • If you had a bigger z scores you can say the group had a bigger difference

9. To report an independent Mann Whitney test in APA format you need to know the U value and ideally the z value and the p value

   • we tend to report the medians
   • for their to be a significant difference the p value needs to be less than .05

10. R (effect size) = Z/√N

    • N=total number of observation
    • different effect sizes mean different things
      • large: >0.5
      • medium: >0.3
      • small: <0.3