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A precursor of counting, the ability to point out each member of a set once and only once, was examined in 58 children aged 23-1-4 years. Performance was positively correlated with age and with ability to count and was negatively correlated with the number of items (3-9) in the set. The effect of arrangement of items depended in part on their similarity. This performance is thought not only to underlie counting but also to measure attention span.
Potter et al. (1968) studied this question.