- What is the Mann Whitney Test?
- Null Hypothesis
- Direct Method
- How to run a Mann Whitney Test in SPSS (Video)
The Mann-Whitney U test is the nonparametric equivalent of the two sample t-test. While the t-test makes an assumption about the distribution of a population (i.e. that the sample came from a t-distributed population), the Mann Whitney U Test makes no such assumption.
The test compares two populations. The null hypothesis for the test is that the probability is 50% that a randomly drawn member of the first population will exceed a member of the second population.
Another option for the null hypothesis is that the two samples come from the same population (i.e. that they both have the same median).
The result of performing a Mann Whitney U Test is a U Statistic. For small samples, use the direct method (see below) to find the U statistic; For larger samples, a formula is necessary. Or, you can use technology like SPSS to run the test.
This method is limited only by how much computation you want to perform. The larger the sample, the more complex the math:
- Name the sample with the smaller ranks “sample 1” and the sample with the larger ranks “sample 2”. Choosing the sample with the smaller ranks to be “sample 1” is optional, but it makes the computation easier.
- Take the first observation in sample 1. Count how many observations in sample 2 are smaller than it. If the observations are equal, count it as one half. For example, if you have ten that are less and two that are equal: 10 + 2(1/2) = 11.
- Repeat Step 2 for all observations in sample 1.
- Add up all of your totals from Steps 2 and 3. This is the U statistic.
- The dependent variable should be measured on an ordinal scale or a continuous scale.
- The independent variable should be two independent, categorical groups.
- Observations should be independent. In other words, there should be no relationship between the two groups or within each group.
- Observations are not normally distributed. However, they should follow the same shape (i.e. both are bell-shaped and skewed left).
Watch the video for the steps:
Can’t see the video? Click here.
Stephanie Glen. "Mann Whitney U Test: Definition, How to Run in SPSS" From StatisticsHowTo.com: Elementary Statistics for the rest of us! https://www.statisticshowto.com/mann-whitney-u-test/
Need help with a homework or test question? With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. Your first 30 minutes with a Chegg tutor is free!
Comments? Need to post a correction? Please post a comment on our Facebook page.