How to Perform a Mann-Whitney U Test in R

The Mann-Whitney U Test, also known as the Wilcoxon Rank-Sum Test, is a non-parametric test used to determine if there are differences between two independent groups on an ordinal or continuous (but not normally distributed) dependent variable. It is the non-parametric alternative to the independent samples t-test.

In this article, we will delve into the underlying principles behind the Mann-Whitney U Test, guide you through the process of preparing your data, describe the steps to perform the test in R, and instruct you on how to interpret the results.

Understanding the Mann-Whitney U Test

When comparing two groups, the Mann-Whitney U Test ranks all the observations from both groups together. The test then compares the sum of ranks in the two groups. The null hypothesis for the test states that the distribution of the dependent variable is the same across the two groups, meaning there’s no difference between them.

Preparing Your Data

Your data should be in a format where one column represents the dependent variable (the measurement or observation) and another column represents the grouping variable (indicating the group each observation belongs to).

Let’s consider a scenario where you wish to compare the test scores of students from two different teaching methods: traditional and online.

# Sample data
set.seed(123)
traditional <- rnorm(25, mean = 70, sd = 10)
online <- rnorm(25, mean = 75, sd = 10)
scores <- c(traditional, online)
group <- factor(rep(c('Traditional', 'Online'), each = 25))

data <- data.frame(scores, group)

Checking Assumptions

For the Mann-Whitney U Test, you need to ensure:

1. Independence of Observations: The observations between groups should be independent.
2. Ordinal Data: The dependent variable should be at least ordinal.
3. Shape of Distributions: While it doesn’t assume normality, the test assumes that the shape of the distributions is the same for both groups.

Performing the Mann-Whitney U Test in R

The Mann-Whitney U Test can be carried out in R using the wilcox.test() function, without setting the paired argument:

# Mann-Whitney U Test
result <- wilcox.test(scores ~ group, data = data)
print(result)

Interpreting the Results

The output of the test in R will provide you with a W statistic and a p-value. Here’s an example of what the output might look like:

Wilcoxon rank sum test

data:  scores by group
W = 1100, p-value = 0.023
alternative hypothesis: true location shift is not equal to 0

Here’s a breakdown of the output:

• W: The W statistic corresponds to the sum of the ranks from one of the groups, usually the group with fewer observations. If the groups are of equal size, R defaults to providing the rank sum for the first group in the order of the data.
• p-value: The p-value will inform you about the significance of the results. A p-value smaller than the significance level (typically 0.05) indicates that you can reject the null hypothesis. In this example, with a p-value of 0.023, we would reject the null hypothesis at the 0.05 significance level.

From this result, we could conclude that there’s a statistically significant difference between the scores of students taught via traditional methods and those taught online.

Conclusion

The Mann-Whitney U Test is a robust non-parametric statistical test that allows for the comparison of two independent groups when the assumptions of the t-test are not met. It’s especially useful for datasets that are not normally distributed or when dealing with ordinal data.

R provides a user-friendly environment to quickly perform this test via the wilcox.test() function. When interpreting results, always pay careful attention to the p-value and compare it to your chosen significance level to make informed decisions regarding the statistical differences between groups.