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When should we use one-tailed hypothesis testing?

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Summary 1. Although one-tailed hypothesis tests are commonly used, clear justification for why this approach is used is often missing from published papers. 2. Here we suggest explicit questions authors should ask of themselves when deciding whether or not to adopt one-tailed tests. 3. First, we suggest that authors should only use a one-tailed test if they can explain why they are more interested in an effect in one direction and not the other. 4. We suggest a further requirement that adoption of one-tailed testing requires an explanation why the authors would treat a large observed difference in the unexpected direction no differently from a difference in the expected direction that was not strong enough to justify rejection of the null hypothesis. 5. These justifications should be included in published works that use one-tailed tests, allowing editors, reviewers and readers the ability to evaluate the appropriateness of the adoption of one-tailed testing. 6. We feel that adherence to our suggestions will allow authors to use one-tailed tests more appropriately, and readers to form their own opinion about such appropriateness when one-tailed tests are used

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