A one-tailed test effectively doubles the p level in that one tail.

Name: A one-tailed test effectively doubles the p level in that one tail.
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Quizplus · Accepted Answer

This is true because in a one-tailed test, all of the p-value is allocated to one tail (either the upper or lower tail), whereas in a two-tailed test, the p-value is split between both tails. Therefore, the p-value in a one-tailed test can be thought of as double the p-value in a two-tailed test for that one specific tail.

A One-Tailed Test Effectively Doubles the P Level in That