Spaced repetition is a memorization technique that works by reviewing question/answer pairs according to a schedule that depends on performance.
Be it software or paper flashcards, there are various techniques to calculate the review schedule. For instance: Leitner, SM0, SM2, ... new techniques appear from time to time. Each new scheduling technique is trying to surpass the others, the goal being to calculate the intervals smartly, for instance so that a question is shown just before the student forgets the fact.
Is there work comparing the "efficiency" of some of those scheduling techniques? (few reviews, well-memorized) Preferably research involving a reasonable number of human experiments.