DX i where xi is the coded value of an independent variable, Xi is the real value of the independent variable, X0 is the real value of an independent variable at the centre point, and DX is the step change value.
The yield of flavonoids was considered as the dependent variable or response. For a central composite rotate design with four independent variables at five levels, 31 experimental runs are required. The actual design of experiments is given in Table 1.