where Y represents the dependent variable to be modeled; b0, bi, bj
and bij represent the coefficients of the model; and Xi and Xj represent
the independent variables. The goodness of fit of the models
obtained was evaluated by R2 (multiple determination coefficient),
the Fischer F-test (and the derived P-values) and the standard errors
of the estimate. Three-dimensional surface plots were drawn to
illustrate the effects of the independent variables on the dependent
ones