How including prior knowledge as a subject variable may change outcomes of learning research

Prior knowledge has a marked effect on learning outcomes. Researchers typically rely on a number of methodologies to control for that factor in learning research, including the use of fictional stimuli and domain-novice subjects. The experiments reported here demonstrate that such methodological controls may be insufficient. In Experiment 1, students read texts about fictional places and events. In Experiment 2, novice students in a cognition course were asked to read several advanced texts. In both experiments, prior knowledge accounted for a large portion of the subjects' posttest performance. The data demonstrate that methodological approaches intended to control for prior knowledge may be insufficient to prevent that variable from influencing learning outcomes. Thus researchers are urged to include measures of prior knowledge in their analyses.