User Guide

9
GLM Multivariate Analysis
After a plot is specified by selecting factors for the horizontal axis, and optionally,
factors for separate lines and separate plots, the plot must be listed in the Plots list.
GLM Multivariate Post Hoc Multiple Comparisons
for Observed Means
Figure 1-6
Post Hoc Multiple Comparisons for Observed Means dialog box
Post hoc multiple comparison tests. Once you have determined that differences exist
among the means, post hoc range tests and pairwise multiple comparisons can
determine which means differ. Comparisons are made on unadjusted values. These
tests are used for between-subjects factors only. The post hoc multiple comparison tests
are performed for each dependent variable separately.
The Bonferroni and Tukey’s honestly significant difference tests are commonly
used multiple comparison tests. The Bonferroni test, based on Student’s t statistic,
adjusts the observed significance level for the fact that multiple comparisons are made.
Sidak’s t test also adjusts the significance level and provides tighter bounds than the
Bonferroni test. Tukey’s honestly significant difference test uses the Studentized
range statistic to make all pairwise comparisons between groups and sets the
experimentwise error rate to the error rate for the collection for all pairwise
comparisons. When testing a large number of pairs of means, Tukey’s honestly
significant difference test is more powerful than the Bonferroni test. For a small
number of pairs, Bonferroni is more powerful.