如果不明白bias和variance的概念, 务必参考下面链接:Gentle Introduction to the Bias-Variance Trade-Off in Machine Learninghttp://machinelearningmastery.com/gentle-introduction-to-the-bias-variance-trade-off-in-machine-learning/Understanding the Bias-Variance Tradeoffhttp://scott.fortmann-roe.com/docs/BiasVariance.html