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proportion of knowledge retained in memory at any given time. Retention is greater than 100% minus the forgetting index. The forgetting index refers to the probability of forgetting at the moment of a repetition while retention refers to the average recall probability between the last and the next repetition. For an exact formula linking the forgetting index and the retention see: Theoretical aspects of SuperMemo. Retention equals 100% minus the forgetting index only when measured on items repeated on a single day (e.g. as displayed in Toolkit : Calendar)