Nonparametric or distributionfree methods have several advantages or benefits. They may be used on all types of data including nominal, ordinal, interval and ratio scaled. They make fewer and less stringent assumptions than their parametric counterparts. Depending on the particular procedure, nonparametric methods may be almost as powerful as the corresponding parametric procedure when the assumptions of the latter are met. When this is not the case, they are generally more powerful.
A parametric method – Consider using when:
A nonparametric method – Consider using if the data is:
Black Belts may have a false sense of security when using nonparametric methods because it is generally believed that nonparametric tests are immune to data assumption violations and the presence of outliers. While nonparametric methods require no assumptions about the population probability distribution functions, they are based on some of the same assumptions as parametric methods, such as randomness and independence of the samples.
In addition, many nonparametric tests are sensitive to the shape of the populations from which the samples are drawn. For example, the 1sample Wilcoxon test can be used when the team is unsure of the population’s distribution but the distribution is assumed to be symmetrical. For the KruskalWallis test, samples must be from populations with similar shapes and equal variances. The KruskalWallis test is more powerful than the Mood’s Median test for data from many distributions, but is less robust against outliers.
Table 1 contains the most commonly used parametric tests, their nonparametric equivalents and the assumptions that must be met before the nonparametric test can be used.
Table 1: Parametric, Nonparametric Equivalents and Assumptions  
Parametric Statistical Test  Nonparametic Equivalents  Nonparametric Data Assumptions 
1Sample zTest or 1Sample tTest  1Sample Sign Test 1Sample Wilcoxon Test  Bivariate random variables are mutually independent. The measurement scale is at least ordinal. Random, independent sample is from a population with a symmetric distribution. 
2Sample tTest  MannWhitney  Mutually independent random samples from two populations that have the same shape, whose variances are equal and a scale that is ordinal. 
Paired tTest  Paired Wilcoxon Test  Random, independent samples are from populations with symmetric distributions. 
OneWay ANOVA  KruskalWallis Test Mood’s Median Test  Random, mutually independent samples are from populations whose distribution functions have the same shape, equal variances. Each sample consists of five or more measures. KruskalWallis is more powerful than Mood’s for data from many distributions, but less robust against outliers. Independent random samples from population distributions that have the same shape. Mood’s Median test is robust against outliers. 
TwoWay ANOVA  Friedman Test  Responses for each of the blocktreatments are from populations whose distribution functions have the same shape and equal variances. Treatments must be randomly assigned within the blocks. 
The figure below provides a roadmap for selecting the appropriate nonparametric method.
Nonparametric methods are essential tools in the Black Belt’s analytic toolbox. When appropriately applied, nonparametric methods are often more powerful than parametric methods if the assumptions for the parametric model cannot be met.


© Copyright iSixSigma 20002018. User Agreement. Any reproduction or other use of content without the express written consent of iSixSigma is prohibited. More »
Comments
For the roadmap above, what are the tests for Same Shape? I have 2 distributions that appear to be skewed in the same way but this is a visual examination. Is there a statistical test to determine fi they are indeed the same shape?
MODIFIED SIGN TEST is structurally designed by Ekene Chukwuonye and Raymond Ezema to take into account the ties in ORDINARY SIGN TEST and makes unnecessarily the assumption that a data be continuous. +2348063867536/+2348069055155