Minitab provides three graphs that help you identify the terms that influence the response: a Pareto chart, a normal plot, and a half-normal plot. These graphs allow you to compare the relative magnitude of the effects and evaluate their statistical significance. Minitab draws effects plots when the model leaves at least 1 degree of freedom for error.
The threshold for statistical significance depends on the significance level (denoted by α or alpha). Unless you use a stepwise selection method, the significance level is 1 minus the confidence level for the analysis. For more information on how to change the confidence level, go to Specify the options for Analyze Definitive Screening Design. If you use backwards selection or stepwise selection, the significance level is the significance level where Minitab removes a term from the model, known as Alpha to remove. If you use forward selection, the significance level is the significance level where Minitab adds a term to the model, known as Alpha to enter. For more information on the choices for the stepwise methods, go to Perform stepwise regression for Analyze Definitive Screening Design
- Select to determine the magnitude and the importance of an effect. The chart displays the absolute value of the standardized effects and draws a reference line on the chart. Any effect that extends beyond this reference line is statistically significant. The α-value for statistical significance is on the graph. The α-value is 0.05 by default when you do not use stepwise regression.
- Select to compare the magnitude and statistical significance of the standardized effects. The fitted line indicates where you would expect the points to fall if the effects were zero. Significant effects have a label and fall toward the left or right side of the graph.
- The normal probability plot displays negative effects on the left side of the graph and positive effects on the right side of the graph.
- Half Normal
- Select or to compare the magnitude and statistical significance of the standardized effects. The fitted line indicates where you would expect the points to fall if the effects were zero. Significant effects have a label and fall toward the right side of the graph.
- The half normal plot displays the absolute value of all effects, positive and negative. Instead of putting negative effects to the left and positive effects to the right, all the significant effects are on the right side, which emphasizes their relative magnitudes.
You can select Display only model terms to display only the terms that are in the model or select Display all terms to display all of the terms in the graphs.