When you’ve learned a subject, it is also easier to answer the question: What is something that, if I forgot it, would indicate a regression in my understanding? In theory (even if Anki doesn’t support it), every answer to that question could be a spaced repetition “card”. For example, maybe every repetition is a new problem of a specific kind that I should be able to solve, and I have to solve it to make sure that I haven’t forgotten how to do it. This is a bit outside of the scope of traditional spaced repetition research, since sometimes seeing the solution to a problem you forgot how to solve doesn’t make you recall how to solve it yourself. Nonetheless, I think this falls under the umbrella of things that have a forgetting curve ( https://en.wikipedia.org/wiki/Forgetting_curve), and thus could be optimized using spaced repetition algorithms.