Six Sigma for Business Excellence: Approach, Tools and Applications

13

Nonparametric Tests

Introduction

Most of the hypothesis tests that we learned about earlier assume normal distribution of data. However, this assumption is sometimes not right. Examples can be: data of project cycle time, lead time, time to repair machines, etc. Nonparametric tests—also known as distribution-free tests—can be performed when the data is clearly not normal and we do not know the shape of the distribution. The power of nonparametric tests is less than parametric tests and confidence intervals are wider.

There are various kinds of nonparametric tests. Some of these are given below:

1. Sign Test
2. Wilcoxon Signed-Rank Test
3. Ranked Sum Mann-Whitney U Test
4. Kruskal-Wallis Test
5. Mood's Median Test
6. Friedman Test
7. Levene's Test

Unless otherwise ...