Use the Compare Means options to perform multiple comparisons of group means. Methods include the Tukey-Kramer all pairs test and the Dunnett’s multiple comparison test with a control. All of these methods use pooled variance estimates for the means. Each Compare Means option, except for the Each Pair Stepwise, Newman-Keuls method, adds comparison circles next to the plot and specific reports to the report window. For more information about comparison circles, see Using Comparison Circles.
Note: Another method for comparing means is ANOM. See Analysis of Means Methods.
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Option |
Description |
Nonparametric Menu Option |
|---|---|---|
|
Each Pair, Student’s t |
Computes individual pairwise comparisons using Student’s t tests. If you make many pairwise tests, there is no protection across the inferences. Therefore, the alpha-size (Type I error rate) across the hypothesis tests is higher than that for individual tests. See Each Pair, Student’s t. |
Nonparametric > Nonparametric Multiple Comparisons > Wilcoxon Each Pair |
|
All Pairs, Tukey HSD |
Shows a test that is sized for all differences among the means. This is the Tukey or Tukey-Kramer HSD (honestly significant difference) test (Tukey 1953; Kramer 1956). This test is an exact alpha-level test if the sample sizes are the same, and conservative if the sample sizes are different (Hayter 1984). See All Pairs, Tukey HSD. |
Nonparametric > Nonparametric Multiple Comparisons > Steel-Dwass All Pairs |
|
With Best, Hsu MCB |
Tests whether the means are less than the unknown maximum or greater than the unknown minimum. This is the Hsu MCB test (Hsu 1996; Hsu 1981). See With Best, Hsu MCB. |
none |
|
With Control, Dunnett’s |
Tests whether the means are different from the mean of a control group. This is Dunnett’s test (Dunnett 1955). See With Control, Dunnett’s. |
Nonparametric > Nonparametric Multiple Comparisons > Steel With Control |
|
Each Pair Stepwise, Newman-Keuls |
Tests whether there are differences between the means using the Studentized range test in a stepwise procedure. This is the Newman-Keuls or Student-Newman-Keuls method (Keuls, 1952). This test is less conservative than a Tukey HSD test. See Each Pair Stepwise, Newman-Keuls. |
none |
Note: If you have specified a Block column, then the multiple comparison methods are performed on data that has been adjusted for the Block means.
For an example showing all of these tests, see Example Contrasting Four Compare Means Tests.