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number associated with each memorized element. It equals to the ratio of the current interval and the previously used interval. If the element has not been repeated yet, i.e. the previous interval is not defined, U-Factor equals the first interval. The greater the U-Factor the greater the increase of the interval between repetitions. For items, U-Factors are determined by SuperMemo Algorithm. For topics, U-Factors are determined by A-Factors. For topics that have already been reviewed at least once, that have not been subject to postpones, U-Factors equal A-Factors. U in U-Factor stands for used interval increase