5. Process Improvement
5.5. Advanced topics
5.5.9. An EDA approach to experimental design
5.5.9.7. |Effects| plot

5.5.9.7.4.

Pattern significance

Look for L-shaped pattern

The |effects| plot has a characteristic horizontally-elongated L-shaped pattern. The vertical arm of the L consists of important factors. The horizontal arm is comprised of unimportant factors. If a factor is important, the bar height will be large and succeeding bar heights may drop off considerably (perhaps by 50 %)--such factors make up the left arm of the L. On the other hand, if a factor is not important, its bar height will tend to be small and near-zero--such factors make up the bottom arm of the L. It is of interest to note where the kink is in the L. Factors to the left of that kink are arguably declared important while factors at the kink point and to the right of it are declared unimportant.

Factor labels

As a consequence of this "kinking", note the labels on the far right margin of the plot. Factors to the left and above the kink point tend to have far-right labels distinct and isolated. Factors at, to the right, and below the kink point tend to have far right labels that are overstruck and hard to read. A (rough) rule-of-thumb would then be to declare as important those factors/interactions whose far-right labels are easy to distinguish, and to declare as unimportant those factors/interactions whose far-right labels are overwritten and hard to distinguish.