http://www.clemson.edu/economics/faculty/wilson/R-tutorial/Introduction.html
https://www.youtube.com/watch?v=cX532N_XLIs&list=PLqzoL9-eJTNBDdKgJgJzaQcY6OXmsXAHU
R tutorial
原文：http://www.cnblogs.com/zyip/p/4440445.html
