http://machinelearningmastery.com/an-introduction-to-feature-selection/ http://machinelearningmastery.com/feature-selection-in-python-with-scikit-learn/ http://stats.stackexchange.com/questions/40576/is-using-the-same-data-for-feature-selection-and-cross-validation-biased-or-not
http://www.slideshare.net/irecsys/matrix-factorization-in-recommender-systems