where, Y is the response; Xi and Xj are variables (i and j rangefrom 1 to k); β0 is the model intercept coefficient; βj, βjj and
βij are interaction coefficients of linear, quadratic and thesecond-order terms, respectively; k is the number ofindependent parameters (k=3 in this study); and ei is the error(Prakash Maran et al. 2013b).