In spaced repetition not all tasks have the same weight
When planning a spaced repetition flow for learning a new concept, not all questions have the same weight nor present the same challenge to remember. Therefore, an algorithm that tries to find the optimum time between exposure to the questions must take into account the variability between cards, and a correct answer does not necessarily mean the card must be pushed to the future the same amount than with other cards.
Questions can be generated by the learners, or by the teachers. The first are the ones who know the at-hand difficulties, but the latter know the final goal of the process, and therefore are able to judge which questions have a higher weight in the student's process.
What is the difference between a question and an exercise? The anki system assumes low answering-time per question, but this is not necessarily the case if we need to think instead of just recall.
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