What defines a one-tailed hypothesis?

A one-tailed hypothesis is characterized by its specification of a direction in which an effect or relationship is expected to occur. This means that it does not just measure whether there is a difference between groups or variables, but rather asserts that one group or variable will be greater than or less than the other, based on theoretical or empirical reasoning.

For example, if a researcher hypothesizes that a new therapy will improve patient outcomes compared to an existing therapy, this hypothesis would be one-tailed because it specifically predicts that the new therapy will lead to better outcomes (and not just different outcomes) than the existing treatment.

In contrast to one-tailed hypotheses, two-tailed hypotheses examine whether there is any significant difference, without speculation on the direction of that difference. Thus, the specification of direction in a one-tailed hypothesis is what fundamentally distinguishes it from other types of hypotheses.