Statistical power is influenced by the direction of the hypothesis. Remember that we can pose a directional or nondirectional hypothesis. In our example, we have been posing a directional hypothesis; we think that college students today have less face-to-face social interaction than previous groups of college students. This is directional because we have specified that they will have less face-to-face interaction. If we thought that students might have more or less face-to-face interaction, we could test this hypothesis with a nondirectional test. We would hypothesize that they were different, rather than that they had less face-to-face interaction. Directional tests are known as one-tailed tests because all of the error is is one tail of the distribution (less than). Nondirectional tests are called two-tailed tests because we must include the possibility that the alternative population could be less than μ or greater than μ.

Directional, or one-tailed, tests are more powerful than nondirectional, or two-tailed, tests. Let's see why. Here is our statistical power graph.