I recently stumbled upon a Wikipedia article about the Leitner system. The Leitner system is a method of studying flashcards where the harder terms are studied more frequently than the easier ones. It was proposed in the 1970's by a German science journalist named Sebastian Leitner. Little did I know, this is very similar to the system that Clustur uses. I had no idea that the concept was so new. Of course, the method has probably been used ever since people have been using flash cards, but I found it interesting that sorting them according to had not been described until the 1970's.

Quoted directly from the Wikipedia article: "The advantage of this method is that you can focus on the most difficult flashcards, which remain in the first few groups. The result is, ideally, a reduction in the amount of study time needed." I hope that Clustur helps users in the same sense.