Null hypothesis usually express the phenomenon of no effect or no difference.
TypeⅠerror is the incorrect rejection of a true null hypothesis. That is, no difference is taken as difference.
Type Ⅱ error is incorrectly retaining a false null hypothesis. That is, difference is taken as no difference
Null hypothesis TypeⅠerror Type Ⅱ error
原文：http://www.cnblogs.com/zhulinmails/p/5829954.html
