How spaced repetition works, and why it beats rereading

You learn a new word on Monday. By Thursday it is gone, and you meet it again as if for the first time. Everyone's memory does this, whatever their talent for languages. Spaced repetition works with that forgetting instead of against it.

Forgetting has a shape

In the 1880s Hermann Ebbinghaus memorised lists of nonsense syllables and measured how much of them he still knew hours and days later. What he found has been confirmed many times since: most of what we forget, we forget soon after learning it, and the loss slows down after that.

Each time you recall something just as it starts to fade, the memory becomes more durable, and the next drop is slower. So the best moment to review a word is when it has started to fade but you can still recall it.

Recalling beats rereading

There are two ways to "go over" a word list:

• read it again;
• cover the answers and try to produce them.

The second one feels harder, and the effort is what makes it work. Researchers call it the testing effect: pulling an answer out of memory strengthens it more than seeing it once more. A flashcard is the simplest tool for it — the question on the front, the answer hidden on the back.

The gaps grow

Put the two ideas together and you get spaced repetition: test yourself, and leave a longer gap after each success. In Brain Cards, a card you answer right every time it comes back is shown on this schedule:

Five reviews, and the word has stayed with you for more than half a year. A card you get wrong comes back sooner instead, so your time goes to the words that need it.

How Brain Cards picks the gaps

Brain Cards schedules every card with FSRS, an open-source scheduling algorithm. It estimates how well you know each card from your past answers and sets the next review for the day your chance of still remembering it falls to about 90%. Every card keeps its own schedule: an easy word moves far ahead, a stubborn one keeps coming back.

That is why a session stays short. It takes the cards that are due first, starting with the ones you are most likely to have forgotten, and fills the rest with new cards. You choose how many cards a session has: ten unless you change it.

Using it

Try to answer before you turn the card. Mark a miss as a miss: a card you call known too early leaves your schedule for weeks, and you meet it again only after it is gone. And come back on the days the cards are due, even for five minutes — the schedule assumes you will.