Following on from briefing I have attached a few reference points that may be of interest.
Here is a link to Bjork's website where he discusses a number of practice methods and his memory model based on cognitive research http://bjorklab.psych.ucla.edu/research.html. This gives a nice overview of current theory.
Interleaved Practice Improves Mathematics Learning
Rohrer looks specifically at using interleaved practice on mathematical problems.
Using Spacing to Enhance Diverse Forms of Learning: Review of Recent Research and Implications for Instruction
This piece by Carpenter reviews literature and attempts to answer the question "how long to wait when spacing?", there is also suggestions on how they could impact teaching in practice.
The Effects of Interleaved Practice
Taylor and Rohrer share results two experiments in this paper comparing the methods of interleaved and blocked practice.
The shuffling of mathematics problems improves learning
Taylor and Rohrer share results two experiments in this paper comparing the methods of interleaved and blocked practice, again.
Retrieval practice: the lack of transfer to deductive inferences
Finally four experiments by Tran and Rohrer exploring the limitations of simply recall.
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