The rationale behind analysis of variance (including analysis of covariance and multiple analyses of variance and covariance) methods is reviewed, and unplanned and planned methods of evaluating differences between means are briefly described. Two advantages of using planned or a priori tests over unplanned or post hoc tests are presented. In addition to increasing power against Type II error rates, planned comparisons force the researcher to plan the research design more thoughtfully. An example illustrating the greater statistical power of planned comparisons is also provided. Three data tables are included, and two figures are appended. (Author/TJH)