What distinguishes a directional hypothesis from a non-directional hypothesis?

A directional hypothesis is characterized by its specificity regarding the expected outcome and the direction of that outcome. It predicts not only that a difference or effect will occur but also specifies the nature of that difference, implying a particular direction (e.g., "Group A will perform better than Group B"). This is opposed to a non-directional hypothesis, which merely predicts that a difference exists without specifying the direction of that difference.

The distinction in terms of the number of tails used in hypothesis testing is crucial here. Directional hypotheses typically employ one-tailed tests, which assess the possibility of an effect in one specific direction, while non-directional hypotheses use two-tailed tests that consider both directions of an effect equally. This tail distinction captures the essence of how researchers approach their predictions in studies and is fundamental in guiding statistical analyses.

In this context, while the specificity of outcomes, prior research, and clarity of the hypothesis statement are important components of hypothesis development, it is the statistical approach indicated by the number of tails that fundamentally differentiates directional hypotheses from non-directional ones.