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:

For the fondue data, the residuals do not appear to be randomly scattered about zero. Evidence exists that there is a pattern in the residuals or curvature in the data. The fondue data was collected using a variety of foods. Shrimp (level 1), apples (level 2), and bread (level 3) were dipped into the fondue. The Residuals versus Food plot indicates that the food dipped into the fondue seems to have a systematic effect on the strength measurements. The residuals for apples are more widely spread than the residuals for shrimp and bread. You may want to include Food in your analysis and evaluate its effect on the flavor.