Residuals

Residual is the difference between the predicted (estimate) and observed (actual) values of the response.

Troubleshooting: Residual plots are useful to examine when troubleshooting your model. Examples of such plots are a plot of the residuals versus the run number, or residuals versus a response or factor value. Any trends that are observed indicate an effect that has not been properly captured in the experiment. Residual plots can be based on either raw values or values that are Studentized. If a few runs have very high residuals, you should examine the runs to check the validity of the data.