Post Hoc Comparisons

The F test used in analysis of variance (ANOVA) is called an omnibus test because it can detect only the presence or the absence of a global effect of the independent variable on the dependent variable. However, in general, we want to draw specific conclusions from the results of an experiment. Specific conclusions are derived from focused comparisons, which are, mostly, implemented as contrasts between experimental conditions. When these comparisons are decided after the data are collected, they are called post hoc or a posteriori analyses. These comparisons are performed after an ANOVA has been performed on the experimental data of interest. In the ANOVA framework, post hoc analyses take two general forms: (a) comparisons that involve all possible contrasts and (b) comparisons that are ...