Select the graphs to display for Analyze Response Surface Design

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 Response Surface 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 Response Surface Design.

Pareto

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.

Normal

Select to compare the magnitude and statistical significance of linear, square, and interaction 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 to compare the magnitude and statistical significance of linear, square, and interaction 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 standardized 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.

Residuals for Plots

Specify the type of residuals to display on the residual plots. For more information, go to Which types of residuals are included in Minitab?.

• Regular : Plot the regular raw residuals.
• Standardized : Plot the standardized residuals.
• Deleted : Plot the Studentized deleted residuals.

Residual Plots

Use residual plots to examine whether your model meets the assumptions of the analysis. For more information, go to Residual plots in Minitab.

• Individual plots: Select the residual plots that you want to display.

  Histogram

  Display a histogram of the residuals.

  Normal plot

  Display a normal probability plot of the residuals.

  Residuals versus fits

  Display the residuals versus the fitted values.

  Residuals versus order

  Display the residuals versus the order of the data. The run order for each data point is shown on the x-axis.

• Four in one: Display all four residual plots together in one graph.

Residuals versus variables

Enter one or more variables to plot versus the residuals. You can plot the following types of variables:

• Variables that are already in the current model, to look for curvature in the residuals.
• Important variables that are not in the current model, to determine whether they are related to the response.