Updating the Debate on One- versus Two-Tailed Tests with the Directional Two-Tailed Test

Abstract

For half a century, methodologists have debated when to use one-and two-tailed tests. But they conducted the debate with scarcely a mention of the little known directional two-tailed test—the only hypothesis test that, properly used, provides for a decision in either direction. In contrast, the traditional two-tailed test assesses nondirectional statistical hypotheses and does not provide for a directional decision. A directional two-tailed test with unequal rejection regions can have virtually the same power as a one-tailed test and, unlike one-tailed tests, it provides for deciding in the unpredicted direction. However, a problem unresolved for one-tailed tests remains for the directional two-tailed test, namely, whether one should create unequal rejection regions just because one has grounds to predict an outcome's direction. Nevertheless, the directional two-tailed test will satisfy research needs much more frequently than will traditional tests and should be adopted as the primary, general-purpose hypothesis test.