Analyze Factorial Design

Residual Plots - Residuals versus the Variables

This graph plots the residuals versus another variable. The residuals should fluctuate in a random pattern around the center line. If the variable is already included in the model, use the plot to determine if you should add a higher-order term of the variable. If the variable is not already included in the model, use the plot to determine if the variable is influencing the response in a systematic way.

Use this plot to look for the following:

This pattern...	Indicates...
Pattern in residuals	The variable is influencing the response in a systematic way
Curvature in the points	A higher-order term of the variable should be included in the model

Example Output

$image\facar_5n.gif$

Interpretation

For the insulation data, the residuals appear to be randomly scattered about zero. No evidence exists that there is a pattern in the residuals or curvature in the data. The plot indicates that machine does not seem to have a systematic effect on the response.

Note	The researchers could have included Machine as a blocking variable in the design and accounted for any influence that machine difference had on the strength measurements.