Nonparametric tests are experiments that do not require the underlying population for assumptions. It does not rely on data referring to any particular parametric group of probability distributions. Nonparametric methods are also called distribution-free tests since they do not have any underlying population.
Nonparametric Tests/ Statistics are Helpful when
- Inferences must be made on categorical or ordinal data
- The assumption of normality is not appropriate
- The sample size is small
Advantages of NonParametric Methods
- Easy application (does not even need a calculator in many cases)
- It can serve as a quick check to determine whether or not further analysis is required
- Many assumptions concerning the population of the data source can be relaxed
- Can be used to test categorical (yes/ no) data
- Can be used to test ordinal (1, 2, 3) data
Disadvantages of NonParametric Methods
- Nonparametric procedures are less efficient than parametric procedures. It means that nonparametric tests require a larger sample size to have the same probability of a type-I error as the equivalent parametric procedure.
- Nonparametric procedures often discard helpful information. That is, the magnitudes of the actual data values are lost. As a result, nonparametric procedures are typically less powerful.
That is they produce conclusions that have a higher probability of being incorrect. Examples of widely used Parametric Tests: include the paired and unpaired t-test, Pearson’s product-moment correlation, Analysis of Variance (ANOVA), and multiple regression.
Note: Do not use nonparametric procedures if parametric procedures can be used.
Some widely used Non-Parametric Tests are:
- Sign Test
- Runs Test
- Wilcoxon Signed Rank Test
- Wilcoxon Rank Sum Test
- Spearman’s Rank Correlation
- Kruskal Wallis Test
- Chi-Square Goodness of Fit Test
Test your knowledge about Non-Parametric: Non-Parametric Quiz