Immediate recall of serial numbers with or without multiple item repetitions

Immediate recall of lists of items in random serial order has been examined in thousands of studies throughout the history of experimental psychology. In most studies, though, there have been no repetitions of items within a list, or occasionally a single repetition. These stimuli differ from the common uses of item series, which often include multiple repetitions (e.g., identification numbers; orders of people at a restaurant table). To begin to understand such cases we presented lists that, in some trial blocks, were constructed with no restrictions on repetitions. Specifically, we examined immediate serial recall of visually-presented, nine-digit lists, either spatially separated into three separate groups of three digits (Experiment 1) or undivided (Experiment 2). Many of the lists included single or multiple repetitions of digits, with repeated digits either adjacent or non-adjacent in an unpredictable manner. We assessed theoretical expectations derived from prior research. Effects of repetition were often helpful but, when repetitions favoured a grouping that conflicted with the presented grouping into threes in Experiment 1, repetition was disadvantageous. We suggest a theoretical analysis in which participants can use presented grouping cues or, when those cues are absent, create their own groupings to exploit repetitions among the stimuli.