The Importance of A Priori Contrasts in Analysis of Variance Research.

The use of planned, or "a priori," and unplanned, or "post hoc," comparisons to isolate differences among means in analysis of variance research is discussed. Planned comparisons typically involve weighting data by sets of "contrasts." Planned comparison offer more power against Type II errors. In addition, they force the researcher to be more thoughtful in conducting research. It is suggested that these two reasons make planned comparisons superior to unplanned comparisons. It is further suggested that orthogonal tests are more useful than non-orthogonal tests. Planned comparisons can be used even when omnibus tests are not statistically significant. (SLD)