Recall of print material benefits from spacing repetitions of that material, an effect often attributed to varied encodings induced by changes in contextual cues. We examined an alternative explanation: retrieving earlier presentations during later presentations strengthens memory traces, the more so the greater the difficulty of such retrieval. In four experiments we found that (a) study-phase retrieval contributes to the benefits of spacing and (b) inducing variation via changes in ad formatting and content can be counterproductive at long spacing intervals, apparently because such changes decrease the likelihood that earlier presentations will be retrieved during later presentations.

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