This view is the primary model selection tool for factorial designs.
By default the red “error line” will be placed such that it represents the smallest 50% of the effects. It is intended to be a visual guide to assist with selecting effects. It can be manually moved to best represent the group of smallest (on the left) effects. The red line will not move automatically until more than 50% of the effects have been selected for the model.
The 50% number can be permanently changed by going to Edit > Preferences > Graphs > Effects Graphs and changing X in the Fit line to X% of smallest effects setting. The look and feel for previous versions is equivalent to setting this to 100%.
Starting at the largest effects (on the right), select terms that fall below or to the right of the red line. The goal is to separate the effects into the large, likely to be repeatable effects and the small, likely to be noise effects. If no effects appear to separate from the red error line, then it is possible that no effects are significant.
Ctrl + right-click will show the x-axis effect values that are currently displayed.
Shift + right-click will show the y-axis percentiles that are currently displayed.
For either of the above, the values shown are copied to the clipboard to be pasted into another program.
D. Cuthbert. Use of half-normal plots in interpreting factorial two-level experiments. Technometrics, 1(4):311, 1959.