APA Dictionary of Psychology

any examination in which two or more quantities are compared in accordance with plans established prior to conducting a research study. For example, even before data are collected, a researcher might hypothesize that two groups given personal instruction would show better mean performance on a task compared to those who receive only written instruction. Thus, they could decide in advance to compare the combined personal instruction groups to the written instruction groups. Also called a priori contrast; planned comparison (or contrast). Compare post hoc comparison.