Introduction
Should you use a two-tailed test or a one-tailed test (or similarly, a confidence interval or 1-sided confidence bound)? For those just learning statistics, or who have had only a little training in the subject, this question comes up fairly often. And there is some conflicting information and advice out there. Most often I’ve seen comments critical of one-sided methods, such as:
The short answer is: Never use one tailed tests.