Dunnett's method compares the mean of each level with the mean of a control level. The control level is usually a standard against which you want to compare all other levels.

This method uses a family error rate (often called familywise error rate) to control the rate of type I error. The family error rate is the probability of making one or more type I errors for the entire set of comparisons. Dunnett's method adjusts the error rate for individual comparisons (individual error rate), based on the family error rate you choose.

The grouping information table highlights the significant and nonsignificant comparisons. The table includes the following information.

·    Method – the multiple comparison method used to construct the family of confidence intervals from which the grouping table is generated.

·    Comparison term – each grouping information table compares levels of one factor.

·    N – the sample size for each factor level.

·    Mean – fitted means sorted in descending order.

·    Grouping – contains columns of letters that group the factor levels. Levels that share a letter are not significantly different. Conversely, if they do not share a letter, the level means are significantly different.

To determine a likely range for the difference between two means, use the interval plot of simultaneous confidence intervals.

Blend 1 is the control group. The grouping information for the paint hardness data shows that only Blend 2 does not belong to group A and, therefore, is significantly different from the control group. Blends 1, 3, and 4 produce equivalent paint hardness while Blend 2 is significantly softer.