3.4.4 Plots of Residuals Versus Other VariablesIf data have been collected on any other variables that might possibly affect the response, theresiduals should be plotted against these variables. For example, in the tensile strength experimentof Example 3.1, strength may be significantly affected by the thickness of the fiber, sothe residuals should be plotted versus fiber thickness. If different testing machines were usedto collect the data, the residuals should be plotted against machines. Patterns in such residualplots imply that the variable affects the response. This suggests that the variable should beeither controlled more carefully in future experiments or included in the analysis.
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