https://cdc.tencent.com/2012/10/17/%E5%87%80%E6%8E%A8%E8%8D%90%E5%80%BC%EF%BC%88nps%EF%BC%89%E7%B3%BB%E5%88%97%E4%B9%8B%E4%B8%80-%E5%9F%BA%E6%9C%AC%E5%8E%9F%E7%90%86%E4%B8%8E%E6%93%8D%E4%BD%9C%E6%A8%A1%E5%9E%8B/
https://www.zhihu.com/question/30887547
