Design matrix evaluation implies that there are no aliases for the quadratic model. In general, a minimum of degrees of freedom 3 and 4 has been recommended for lack-of-fit and pure error, respectively. Therefore, degrees of freedom obtained in this study ensured a valid lack of fit test (Table 5).
Table 5 Degrees of freedom for evaluation
Model 14
Residuals 15
Lack of Fit 10
Pure Error 5