Friedman Test

The Friedman test is a non-parametric statistical test used to detect differences across three or more related samples - for example, the same participants rating three interface conditions. It ranks each participant's responses across conditions and tests whether the rank sums differ significantly. Accessibility and HCI studies use it routinely for within-subjects designs where the outcome is ordinal (Likert ratings) or not normally distributed, and it is typically followed by pairwise Wilcoxon signed-rank tests with a Bonferroni correction to identify which specific conditions differ.