“…Concerning the validation methods, 25 of the selected studies use them to assess the prediction accuracy of ECEV models. As can be seen in Figure 10, the cross-validation method is the most frequently used in 15 studies [36,[47][48][49]64,71,102,106,109,111,115,120,121,160,164], followed by 10-fold cross-validation in 10 studies [41,43,51,52,83,103,141,147,165,170], 5-fold cross-validation in 6 studies [91,94,108,152,158,167], then 3-fold cross-validation in 3 studies [92,93,152]. Less studied cross-validations methods are not shown in Figure 10, such as 1-2-4-fold cross validation [152], 6-fold cross validation [137], 8-fold cross validation [146], 9-fold cross-validation [122], and leave-one-out cross-validation [83] which are in only 1 study each.…”