Compared efficiency of different spaced-repetition memorisation techniques 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.
A quick Pubmed search shows a couple of potential useful papers. The more promising:


Karpicke, JD., &amp; Bauernschmidt, A. (2011) Spaced retrieval: absolute spacing enhances learning regardless of relative spacing J Exp Psychol Learn Mem Cogn. 37(5):1250-7
Toppino, TC., Kasserman, JE., &amp; Mracek WA. (1991) The effect of spacing repetitions on the recognition memory of young children and adults J Exp Child Psychol. 51(1):123-38.