One-tailed tests: Let's do this (responsibly).

Abstract

When preregistered, one-tailed tests control false-positive results at the same rate as two-tailed tests. They are also more powerful, provided the researcher correctly identified the direction of the effect. So it is surprising that they are not more common in psychology. Here I make an argument in favor of one-tailed tests and address common mistaken objections that researchers may have to using them. The arguments presented here only apply in situations where the test is clearly preregistered. If power is truly as urgent an issue as statistics reformers suggest, then the deliberate and thoughtful use of preregistered one-tailed tests ought to be not only permitted, but encouraged in cases where researchers desire greater power. One-tailed tests are especially well suited for applied questions, replications of previously documented effects, or situations where directionally unexpected effects would be meaningless. Preregistered one-tailed tests can sensibly align the researcher's stated theory with their tested hypothesis, bring a coherence to the practice of null hypothesis statistical testing, and produce generally more persuasive results. (PsycInfo Database Record (c) 2024 APA, all rights reserved).