Purpose
The Main Effects Plot is used to graphically compare the level of a process output variable at various states of process "factors", to gain an understanding of the main effect of a change in the factor on the output.
Anatomy
Main Effects Plot of the Means
Terminology
A. The Output variable being studied.
B. The Factors whose effect on the output is being studied.
C. The Mean of the Output variable at different levels (or values) of Factor 1 (more than one data point may have been collected for each level of the factor, so the Mean is used ).
D. The Mean of the Output variable at different levels (or values) of Factor 2.
E. The grand Mean of all the output values.
F. The Levels of the Factors.
Major Considerations
The best tool for creating a Main Effects Plot is Minitab, using the They can be used to compare many Factors at one time.
A relatively flat plotted line indicates that as the Factor changes value, it has little effect upon the output, while a line that has a lot of up and down movement indicates that as the Factor changes, it has a greater "main effect".
Application Cookbook
1. Gather data and present in tabular form, with output values being matched with corresponding levels of the chosen Factors.
2. When more than one output value is presented for each factor, calculate the Mean of the output value for each Factor.
3. Calculate the Grand Average of all the output values, and plot it as a straight line on all Main Effects plots.
4. For each Factor, plot the Mean of the output values at each level of the specific Factor.
5. If more than one Factor is being analyzed, the plots for the different Factors should be presented together for comparison.
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