Fig. 5

The convergence of the MF trend, MF and OK models of \(c_2\). In c–h, the left, the middle and the right columns show the values of \(c_{2}^{MF}\), \({\widetilde{c}}_{2}^{MF}\) (see Eq. (9)) and \({\widetilde{c}}_{2}^{OK}\). The points in the DoE are represented as white dots