In research studies involving two groups of participants (e.g., experimental group vs. control group), the null hypothesis always predicts that there will be no differences between the groups being studied (Kazdin, 1992).
If the hypothesis simply predicts that there will be a difference between the two groups, then it is a nondirectional hypothesis (Marczyk, DeMatteo and Festinger, 2005). It is nondirectional because it predicts that there will be a difference but does not specify how the groups will differ.
If, however, the hypothesis uses so-called comparison terms, such as “greater,” “less,” “better,” or “worse,” then it is a directional hypothesis. It is directional because it predicts that there will be a difference between the two groups and it specifies how the two groups will differ (Marczyk, DeMatteo and Festinger, 2005).
Kazdin, A. E. (1992). Research design in clinical psychology (2nd ed.). Boston: Allyn & Bacon.
Marczyk, G., DeMatteo, D., & Festinger, D. (2005). Essentials of research design and methodology. Hoboken, NJ: John Wiley & Sons.