The test of a given statistical hypothesis in which a value of the statistic that is either sufficiently small or sufficiently large will lead to rejection of the hypothesis tested.

The statistical tables for Z and for t provide critical values for both one- and two-tailed tests. That is, they provide the critical values that cut off an entire α region at one or the other end of the sampling distribution as well as the critical values that cut off the 1/2 α regions at both ends of the sampling distribution.