Lesson 7 of 7 · 12 min
Parametric vs nonparametric tests
Parametric tests are about parameters and lean on distribution assumptions; when those assumptions fail, the data are ranks, or the question is not about a parameter, use a nonparametric test.
In short
- A parametric test concerns a population parameter (mean, variance) and relies on specific distributional assumptions, usually normality.
- A nonparametric test is not concerned with a parameter, or makes only minimal assumptions about the population.
- Four reasons to go nonparametric: assumptions not met, outliers, data given as ranks (ordinal scale), or a hypothesis that is not about a parameter.
- Alternatives: single mean → Wilcoxon signed-rank; paired differences → Wilcoxon signed-rank or sign test; two independent means → Mann–Whitney U (Wilcoxon rank sum).
- When the parametric assumptions hold, the parametric test is preferred because it usually has more power.
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