In previous works, QSRR were often used not for practical prediction of RI, but for characterization of SP in order to draw some conclusions about the nature of retention mechanisms (see Fig. ). However, it is not usually tested whether the selection of MD is reproducible with small changes to the data set. If the conclusions characterize the SP in general, then these conclusions should not be altered if the data set (that plays the role of a probe) is slightly changed: for example, several molecules are removed. Thus, the procedure of MD selection was repeated many times and each time Also, the accuracy of the model (and of the method in general) depends on the set of selected features. Typically, in QSRR works, a set of MD is selected only once and then the accuracy of the model is carefully investigated (see Fig. ) using cross-validation or one-leave-out approaches.