The selection of a one or two-tailed t-test must be made before the experiment is performed. It is not “cricket” to find a that tOBS = -1.92, and then say “I really meant to do a one-tailed t-test.” Because reviewers of articles submitted for publication are sometimes suspicious when a one-tailed t-test is done, the recommendation is that if there is any doubt, a two-tailed test should be done.（来自统计书本）
总结一下：
1，有很强的证据表明一方比另一方大或者小（有大量的先验知识）；
2，只对一侧的数据感兴趣。
其它情况都要用双尾。