where V s and V t are the already-selected feature set and to-beselected feature set, respectively, and m and n are the sizes of these two feature sets, respectively. The earlier a feature is selected, the better it would be though of. Finally, we can get an ordered feature list with a rank for every feature to indicate its importance in the feature set. In our study, the mRMR program is obtained from: http://research. janelia.org/peng/proj/mRMR/index.htm.