Modeling list-strength and spacing effects using version 3 of the retrieving effectively from memory (REM.3) model and its superimposition-of-similar-images assumption.

Shiffrin and Steyvers (1997) introduced a model of recognition memory called retrieving effectively from memory (REM) and successfully applied it to a number of basic memory phenomena. REM incorporates differentiation, wherein item repetitions are accumulated in a single mnemonic trace rather than separate traces. This allows REM to account for several benchmark findings, including the null list-strength effect in recognition (Ratcliff, Clark, & Shiffrin, 1990). The original REM treated massed and spaced repetitions identically, which prevents it from predicting a mnemonic advantage for spaced over massed repetitions (i.e., the spacing effect). However, Shiffrin and Steyvers discussed the possibility that repetitions might be represented in a single trace only if the subject identifies that the repeated item was previously studied. It is quite plausible that subjects would notice repetitions more for massed than for spaced items. Here we show that incorporating this idea allows REM to predict three important findings in the recognition memory literature: (1) the spacing effect, (2) the finding of slightly positive list-strength effects with spaced repetitions, as opposed to massed repetitions or increased study time, and (3) list-strength effects that have been observed using very large strong-to-weak ratios (see Norman, 2002).