Using the Fit Mean command, you can add a horizontal line to the scatterplot that represents the mean of the Y response variable. You can start by fitting the mean and then use the mean line as a reference for other fits (such as straight lines, confidence curves, polynomial curves, and so on).
Example of Fit Mean
Error sum of squares for the simple mean model. Appears as the sum of squares for Error in the analysis of variance tables for each model fit.
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