Two-sample Wilcoxon Test
The Wilcoxon rank sum test is used to test whether two sets of observations come from the same distribution. The alternative hypothesis is that the observations come from distributions with identical shape but different locations. Unlike the two-sampled t-test, this test does not assume that the observations come from normal (Gaussian) distributions.
This test is equivalent to the Mann-Whitney test. For paired data, specify Signed Rank as the Type of Rank Test.
To perform a two-sample Wilcoxon test
Choose Statistics Compare
Samples
Two
Samples
Wilcoxon
Rank Test. The
dialog shown below appears.
The two-sample Wilcoxon test has the following options:
Data
Data Set
Select a data set from the dropdown list or type the name of a data set. You can also type into the Data Set edit field any expression that evaluates to a data set.
Variable 1
Specify a column as the first sample, when the data set does not have a grouping indicator.
Variable 2
Specify another column as the second sample, when the data set does not have a grouping indicator.
Variable 2 is a Grouping Variable
Select if one column in the data set is a grouping indicator that categorizes cases into two groups. In this case, select the response from Variable 1 and the indicator from Variable 2.
Test
Type of Test
Choose between Rank Sum and Signed Rank. The Signed Rank test is not available when Variable 2 is a Grouping Variable is selected.
Hypotheses
Mean Under Null Hypothesis
Enter the difference between the assumed population means of Variable x and Variable y.
Alternative Hypothesis
Specify the alternative hypothesis. For example, to perform a one-sided test against the alternative hypothesis that the mean of Variable 1 is greater than the mean of Variable 2, select greater from the list box.
Options
Use Exact Distribution
Select this to use the exact distribution of the test statistic to compute the p-value.
Continuity Correction
Select this to use a continuity correction in the normal approximation to the distribution of the test statistics.
Results
Enter the name for the object in which to save the results of the analysis.
Print Results
Select this to print out the results of the analysis in the designated output window.
Related S-PLUS language functions
wilcox.test, t.test