One-Sided Significance Tests

Sometimes, two-sided tests are called “two-tailed” tests! (I’ll get my coat!) Source:

(1) Researchers should halve two-sided p values when making directional claims

Researchers sometimes conduct two-sided significance tests and then use the resulting two-sided p values to make directional claims. I argue that this approach is inappropriate because two-sided p values refer to non-directional hypotheses, rather than directional hypotheses.

(2) Researchers should not halve their alpha level when using two one-sided tests

I also argue that, if you use two one-sided tests to test two directional null hypotheses, then it’s not necessary to adjust your alpha level to compensate for multiple testing, because your decision about rejecting each directional hypothesis is based on a single test result, rather than multiple test results.



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Mark Rubin

Mark Rubin

Interested in issues connected with the replication crisis in science.