User Guide

497
Nonparametric
Te s t s
Figure 34-14
Two-Independent-Samples Tests dialog box
E Select one or more numeric variables.
E Select a grouping variable and click Define Groups to split the file into two groups
or samples.
Two-Independent-Samples Test Types
Test Type. Four tests are available to test whether two independent samples (groups)
come from the same population.
The Man
n-Whitney U test is the most popular of the two-independent-samples
tests. It is equivalent to the Wilcoxon rank sum test and the Kruskal-Wallis test
for two groups. Mann-Whitney tests that two sampled populations are equivalent
in loca
tion. The observations from both groups are combined and ranked, with the
average rank assigned in the case of ties. The number of ties should be small relative
to the total number of observations. If the populations are identical in location, the
ranks
should be randomly mixed between the two samples. The number of times a
score from group 1 precedes a score from group 2 and the number of times a score
from group 2 precedes a score from group 1 are calculated. The Mann-Whitney U
stat
istic is the smaller of these two numbers. The Wilcoxon rank sum W statistic,
also displayed, is the rank sum of the smaller sample. If both samples have the