Figure 1.

Flowchart of two different feature selection strategies. Left panel: all features were used for the feature selection method performed outside of CV, and then selected fixed features were used to evaluate the prediction performance, at which time CV was applied. Right panel: contrary to the feature selection method performed outside of CV, CV was applied in the stage of feature selection for the feature selection method performed within CV, where the features in the training set were only used for feature selection, and then the corresponding features in the test set were used to evaluate the prediction performance.