Tests for Two Independent Samples

There are four non-parametric tests for cases involving two independent samples. These tests are:

• The Mann-Whitney U test
• The Wald-Wolfowitz Runs test
• The Kolmogorov-Smirnov Z test
• The Moses Extreme Reactions test

The Mann-Whitney U test in the tests for two independent samples is an alternative form of the t-test.  It is widely used to test whether or not two independent samples are significantly different.  In SPSS, the Mann-Whitney U test in the tests for two independent samples is done by selecting “analyze” from the menu, then clicking on “Nonparametric Tests.”  After this, select “legacy dialogs” and then “2 Independent Samples” and select the “Mann-Whitney U” option from the Test Type option.

The Wald-Wolfowitz Runs test checks for significant differences between two independent samples of an ordinal variable. In SPSS, you conduct the Wald-Wolfowitz Runs test by selecting “Nonparametric Tests” from the Analyze menu, then choosing “Legacy Dialogs,” followed by “2 Independent Samples.”  Finally, select the “Wald-Wolfowitz runs” option from the Test Type option.

The Kolmogorov-Smirnov Z test checks if the maximum absolute difference in the overall distribution of two groups is significant. In SPSS, you perform the test by selecting “Nonparametric Tests” from the Analyze menu, clicking “Legacy Dialogs,” and then choosing “2 Independent Samples.”  Finally, select the “Kolmogorov-Smirnov Z” option from the Test Type option.