The spacing effect refers to the finding that memory for repeated items improves when the interrepetition interval increases. To explain the spacing effect in free-recall tasks, a two-factor model has been put forward that combines mechanisms of contextual variability and study-phase retrieval (e.g., Raaijmakers, 2003; Verkoeijen, Rikers, & Schmidt, 2004). An important, yet untested, implication of this model is that free recall of repetitions should follow an inverted u-shaped relationship with interrepetition spacing. To demonstrate the suggested relationship an experiment was conducted. Participants studied a word list, consisting of items repeated at different interrepetition intervals, either under incidental or under intentional learning instructions. Subsequently, participants received a free-recall test. The results revealed an inverted u-shaped relationship between free recall and interrepetition spacing in both the incidental-learning condition and the intentional-learning condition. Moreover, for intentionally learned repetitions, the maximum free-recall performance was located at a longer interrepetition interval than for incidentally learned repetitions. These findings are interpreted in terms of the two-factor model of spacing effects in free-recall tasks.

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Experimental Psychology
Department of Psychology

Verkoeijen, P.P.J.L, Rikers, R.M.J.P, & Schmidt, H.G. (2005). Limitations to the spacing effect: Demonstration of an inverted u-shaped relationship between interrepetition spacing and free recall. Experimental Psychology, 52(4), 257–263. doi:10.1027/1618-3169.52.4.257