I remember a documentary about learning things to kids, and in order to optimise learning efficiency the optimal ratio for difficult questions should be 1/4. So

I remember a documentary about learning things to kids, and in order to optimise learning efficiency the optimal ratio for difficult questions should be 1/4. So the children continue to feel good about themselves, while still improving. So I guess this ratio about easy vs difficult question should be a parameter in such spaced repetition algorithms.