The mix-up regarding mixed and unmixed lists in spacing-effect research

J. W. Hall (1992) proposed that participants in spacing-effect experiments use massed repetitions as an opportunity to study previously presented item s. He provided apparent evidence for this displaced-rehearsal strategy in a free-recall experiment involving unmixed lists consisting of all massed or all spaced repetitions. He argued that this strategy produces artifactual spacing effects when experiments involve mixed lists containing both massed and spaced repetitions. This raises the specter that much of the spaced-re petition literature might be contaminated by artifactual results because mo st experiments have used mixed-list designs. The authors replicated Hall's experiment and extended it by including critical control conditions. Althou gh Hall's results are replicable, they are primarily attributable to factor s that were uncontrolled in his study. There seems to be no compelling reas on to question spacing effects obtained with mixed-list designs.