(2016-02-15, 5:52 pm)ariariari Wrote: I'm not sure how they picked 21 days as the cutoff.
It's arbitrary, but you can kind of reverse engineer it. Assuming the defaults most regular cards which you answer only "good" on will eventually be seen at an interval of between 14-16 days and then the next one is around 38-41. That makes the midpoint of those 26-28 so we probably want a number slightly less than 26 but far more than 16. It's hard to say why 21 specifically was picked but it does fall into a nice period of exactly three weeks and is slightly conservative in case you had to fail a card once or twice. Damien has a lot of data he can query to make guesses like that so it was probably also influenced on whatever was stored in AnkiWeb at the time he was trying to come up with a sensible number.
Like Trippy wrote the specific number is not nearly important as the fact that you have a consistent metric.
Edited: 2016-02-16, 1:31 am