where, Fmax/min ¼ maximizing or minimizing objective function, Xi ¼ input variables, Ci ¼ coefficients of the model. To define objective functions, attention must be paid to certain important aspects. If a linear model (i.e. linear regression) can show the relation between input parameters and output appropriately, the variance inflation factor (VIF) should be examined. The value of VIF shows multicollinearity, and parameters with VIFs greater than 5 need to be excluded from the model (Tabatabaie et al., 2013).