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 paint hardness 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 the temperature at which the samples were cured does not seem to have a systematic effect on the response.