Nonparametrics Statistics Notes - Wilcoxon Matched Pairs Test

The Wilcoxon matched pairs test is a nonparametric alternative to the t-test for dependent (correlated) samples. Select Comparing two dependent samples (variables) from the Nonparametric Statistics Startup Panel - Quick tab to display the Comparing two variables dialog box, in which you specify variables from two lists. Each variable from the first list will be compared with each variable from the second list. This is the same arrangement of data that is expected for the t-test (dependent samples) in the Basic Statistics module.

Assumptions and interpretation

The procedure assumes that the variables under consideration were measured on a scale that allows the rank ordering of observations based on each variable (i.e., ordinal scale) and that allows rank ordering of the differences between variables (this type of scale is sometimes referred to as an ordered metric scale, see Coombs, 1950). Thus, the required assumptions for this test are more stringent than those for the Sign test. However, if they are met, that is, if the magnitudes of differences (e.g., different ratings by the same individual) contain meaningful information, then this test is more powerful than the Sign test. In fact, if the assumptions for the parametric t-test for dependent samples are met, then this test is almost as powerful as the t-test.

See Comparing Two Variables - Quick tab for further details.