Every word you study carries a mastery level from 0 to 5, and each level has one review interval attached to it. Answer correctly and the word moves up a level, then waits for that level's interval. Climbing from zero, the gaps you actually get are 1 day, 2 days, 4 days, 7 days, then 14 days. Answer wrongly and the word drops a level and comes back immediately, with no interval at all.
The full ladder
The intervals are stored as a list of six numbers, one per mastery level, and the interval you get is the one belonging to the level you land on after answering:
- Reach mastery 1 — next review in 1 day
- Reach mastery 2 — next review in 2 days
- Reach mastery 3 — next review in 4 days
- Reach mastery 4 — next review in 7 days
- Reach mastery 5 — next review in 14 days
Mastery 5 is the ceiling, so a word you keep getting right settles into a fortnightly rhythm rather than drifting out to months. There is a sixth number in the list, one day for mastery 0, but a correct answer can never land you there, because correct always moves you up. It is a floor, not a step you climb through.
The gaps roughly double at first and then round to human units. Four days becomes a week rather than eight days, and a week becomes a fortnight. Whether that matters to memory is not something this schedule can prove, but it does mean a word met on a Monday tends to come back on a Monday.
What a wrong answer does
It drops the word one mastery level, with zero as the floor, and marks it due right now. It does not get the interval for its new level. A word you have just failed is a word you do not know, and making you wait a day to find that out again would be the wrong answer to the wrong question.
This is the part that makes the number honest. Mastery is not a count of how many times you have seen a word. It moves down as readily as it moves up, so a word sitting at 4 is a word you have got right four more times than you have got it wrong, recently, at spacing the app chose rather than spacing you chose.
Why not one schedule for the whole set?
Because you do not know every word in a set equally well. Fifty words imported together will separate within a few sessions: the cognates and the ones you already half-knew climb to mastery 4 and disappear for a week, while six awkward ones keep dropping back to zero and keep reappearing.
A schedule applied to the whole set would show you all fifty on the same day, which spends most of the session on words that did not need it. Per-word scheduling is what makes a twenty-word session mostly the words you are actually failing.
Answering the same word twice in ten minutes
It does not count twice. A correct answer given within ten minutes of the last one that counted is recorded, but it does not raise mastery and it does not push the review date out.
The reason is that the answer is still sitting in your working memory. Recalling something you read ninety seconds ago is not evidence that you will recall it on Thursday, and without that guard, replaying one round three times in a row would walk every word to mastery 5 and schedule it a fortnight away having never once been recalled cold.