Updating the debate on one- versus two-tailed tests with the directional two-tailed test

For half a century, methodologists have debated wen to use one-and two-tail ed tests. But they conducted the debate with scarcely a mention of the litt le known directional two-tailed test-the only hypothesis test that, properl y used, provides for a decision in either direction. In contrast, the tradi tional two-tailed test assesses nondirectional statistical hypotheses and d oes not provide for a directional decision. A directional two-tailed test w ith unequal rejection regions can have virtually the same power as a one-ta iled test and, unlike one-tailed tests, it provides for deciding in the unp redicted direction. However, a problem unresolved for one-tailed tests rema ins for the directional two-tailed test, namely, whether one should create unequal rejection regions just because one has grounds to predict an outcom e's direction. Nevertheless, the directional two-tailed test will satisfy r esearch needs much more frequently than will traditional tests and should b e adopted as the primary, general-purpose hypothesis test.