Spacing between two attempts to learn one item greatly increases the probability that it will be remembered later. We first note that a number of theories that explain this phenomenon also predict that spacing should improve the probability of remembering at least one of two different items each studied once. We then report results of two experiments designed to test this prediction. Subjects studied a series of 300 words. After a short rest they were tested for recognition, or-in the second experiment-free recall and then recognition. As usual a substantial spacing effect was observed for single words presented twice. In contrast the probability of correct recognition or recall of at least one word of a pair of two different words, each presented once, did not depend on their spacing in the study series.
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