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A Mann-Whitney U test is a non-parametric statistical test used to compare the difference between two independent groups. To report the results of a Mann-Whitney U test, one should include the sample size and the calculated U value, along with the significance level and the p-value. For example, “A Mann-Whitney U test was conducted to compare the test scores of two groups of students. The results showed a significant difference between the groups (U = 75, p < 0.05).” This indicates that there is a significant difference between the two groups, with a U value of 75 and a p-value less than 0.05.
A Mann-Whitney U test is used to compare the differences between two independent samples when the sample distributions are not normally distributed and the sample sizes are small (n <30).
When reporting the results of a Mann-Whitney U test, we always use the following general structure:
- A brief description of the independent and dependent variable.
- The overall z-value of the test and the corresponding p-value.
Here is the exact wording we can use:
A Mann-Whitney U test was performed to compare [response variable of interest] in [group 1] and [group 2].
There [was or was not] a significant difference in [response variable of interest] between [group1] and [group2]; z = [z-value], p = [p-value].
The following example shows how to report the results of a Mann-Whitney U test in practice.
Example: How to Report Results of Mann-Whitney U Test
Suppose researchers want to know if a new fuel leads to a change in the average mpg of a car.
To test this, they conduct an experiment in which they measure the mpg of 12 cars with the new fuel and 12 cars with normal fuel.
To determine if there is a significant difference between the two groups, they perform a Mann-Whitney U test in SPSS and receive the following results:
The most important numbers in the output are the Z test statistic and the Asymptotic 2-tailed p-value:
- Z test statistic: -1.279
- p-value: .201
Since the p-value is not less than 0.05, we . We do not have sufficient evidence to say that the true mean mpg is different between the two groups.
Here is how we would report the results of this Mann-Whitney U test:
A Mann-Whitney U test was performed to compare average miles per gallon between cars that received a new fuel and cars that received regular fuel.
There was not a significant difference in average miles per gallon between between cars that received a new fuel and cars that received regular fuel; z = -1.279, p = .201.
Note: As a rule of thumb, we round p-values in the report to three decimal places. However, depending on your industry it may be standard practice to round to a different number of decimal places.
Additional Resources
The following tutorials provide additional information about the Mann-Whitney U test: