where Y is the predicted response, ˇ0 is a constant and ˇi, ˇii and ˇij are the linear, quadratic and interactive coefficients, respectively.
Xi and Xj indicate levels of the variables. In order to estimate the coefficients of the polynomial by least square fitting method,
Minitab 16 software was used and validity of the model was investigated by analysis of variance (ANOVA).