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Statistic, Unbiased; Parameter; Variation.

Explanation: Spaced Repetition System  is a learning system that allows us to memorize information (a foreign word) efficiently. It seeks to minimize the number of repetitions required to retain information. For this, the SRS schedules the review for when we are about to forget, as it is the ideal time to optimally review, minimizing the number of repetitions. Thus, neither too many nor too little revisions.