Entropy (information gain)

http://zh.wikipedia.org/wiki/%E7%86%B5_(%E4%BF%A1%E6%81%AF%E8%AE%BA)

http://en.wikipedia.org/wiki/Information_gain_in_decision_trees

http://www.autonlab.org/tutorials/infogain11.pdf

http://homes.cs.washington.edu/~shapiro/EE596/notes/InfoGain.pdf