The normal probability plot indicates that the points on this plot are formed a nearly linear pattern (Fig. 2 (a)). Therefore, the normal distribution is a good model for this data set. Random scattering of the points of internally studentized residual (the residual divided by the estimated standard deviation of that residual) versus predicted values between −3 and +3 emphasizes highly accurate prediction of the experimental data through the derived quadratic model (Fig. 2 (b)).
Fig 2 The plot of: (a) Normal Plot of Residuals; (b) residuals vs Predicted Response; (c) predicted vs Actual values; (d) Residuals vs Run Order