The comparison procedures allow you to test the mean difference between pairs of groups. If you do not need to compare all groups, consider using a method that compares specific groups because these methods are more powerful. For example, using Tukey's all-differences approach is inefficient when Dunnett's or Hsu's MCB is suitable, because Tukey's confidence intervals are wider and the hypothesis tests less powerful for a given family error rate. For the same reasons, Hsu's MCB is superior to Dunnett's if you want to eliminate levels that are not the best and to identify those that are best or close to the best. The choice between Tukey's and Fisher's depends on whether you want to specify the family or individual error rate.
If you do not assume equal variances in the Options dialog box, you must use the Games-Howell method.
- Error rate for comparisons
- Enter an error rate between 0.001 and 50. The default is 5. The error rate is the individual error rate for Fisher's method and the family error rate for all other methods.
- Comparison procedures assuming equal variances
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- Tukey
- Compare all pairs of groups, while controlling the simultaneous confidence level. Tukey's provides confidence intervals that are less precise and hypothesis tests that are less powerful than either Dunnett's or Hsu's MCB.
- Fisher
- Compare all pairs of groups, while controlling the individual error rate. Fisher's method is less common than Tukey's because it does not control the simultaneous confidence level, which often decreases to unacceptable levels.
- Dunnett
- Compare the treatment groups to a control group, while controlling the simultaneous confidence level. From Control group level, select the group you want to compare the other groups to. For more information, go to What is Dunnett's method for multiple comparisons?.
- Hsu MCB
- Identify the groups that are the best. Hsu's MCB compares each group to the group with either the largest or the smallest mean, while controlling the simultaneous confidence level. From Best, select one of the following:
- Largest mean is best: Compare each group to the group with the largest mean.
- Smallest mean is best: Compare each group to the group with the smallest mean.
- Comparison procedure not assuming equal variances
- This option only appears when you deselect Assume equal variances in the Options sub-dialog box.
- Games-Howell
- Compare all pairs of groups, while controlling the simultaneous confidence level.
- Results
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- Interval plot for differences of means
- Display confidence intervals for the differences between the group means for each selected comparison method. Use the confidence intervals to determine likely ranges for the differences and to assess the practical significance of the differences.
- Grouping information
- Display a table that highlights the significant and nonsignificant comparisons for each selected comparison method. No grouping information table is presented for Hsu's MCB.
- Tests
- Display the hypothesis test form of the comparison output, which includes the differences of the means and the adjusted p-values.
