In the Fit Least Squares report, the Compare Slopes option appears when there is one nominal term, one continuous term, and their interaction effect for the fixed effects. This option produces a report that enables you to compare the slopes in an analysis of covariance (ANCOVA) model. The report compares the slopes of each level of the interaction effect to the overall slope. The comparison uses analysis of means (ANOM) with the overall average. For more information about the analysis of means (ANOM) report, see Comparisons with Overall Average.
The overall average slope is a weighted average of the slopes, where the weights are inversely proportional to the variances of the slope estimates. These variances are the squared values of the Std Error column in the Differences from Overall Average Slope table.