where Y is the response. X , X are input variables. b is the intercept.
i1⁄41
ij0
bi is a linear coefficient. bii is a squared coefficient. bij is an interac- tion coefficients, and e is an error term. CCD has 2k factorial exper- iments where k represents the number of independent factor. Two parameters, including the central point (coded as 0) and the dis- tance (a) from the axial point to central point, was decided before designing the experiment. The central point experimental combina- tion was replicated to check the reproducibility of experiments. Based on the definition, a = (2k)1/4. For two-factor design, a i