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interval (inter-repetition interval)
period of time between successive repetitions. In incremental reading, intervals are also periods between the review of topics. Initially, repetitions occur in intervals ranging from 1-15 days; however, with time, intervals can reach well into thousands (which corresponds to decades). Intervals in SuperMemo are measured in days. Occasionally they may be displayed as seconds (if they are very short), or years (if are very long), or with the use of other units