SuperMemo uses a learning interval determination algorithm called SM-2 algorithm (*23). It is now widely used for learning software developed after that. The concept of "easiness factor"(EF) introduced by this algorithm is interesting.

In this algorithm,

You review newly added cards and test after one day

If you answer correctly on a card, the review interval for the card goes six days. You review the card after 6 days.

After that, the review interval of the correctly corrected card goes multiplication the previous review interval by the easiness factor EF.

The easiness factor (EF) factor defaults to 2.5, varying from 1.3 to 2.5. When you answered correctly, you tell the system it is easy or difficult. The easiness factor of the card increase or decrease by the information. If you cannot answer correctly, EF does not change, and the review interval goes to 1 day.

For easy questions, the interval spreads by 2.5 times. So the review interval expands to roughly 6 days, 15 days, 5 weeks, 3 months, 8 months, 1 year and a half, and 4 years. If you correctly answer 7 times, next review is four years later.