Spaced repetition and FSRS, explained

Spaced repetition shows you a word again at growing intervals, timed for just before you would forget it. FSRS, the Free Spaced Repetition Scheduler, decides that timing by estimating how stable your memory of each card is and how likely you are to recall it today, then schedules the next review for when that chance drops to a target.

What is spaced repetition?

Spaced repetition is a way of reviewing that puts growing gaps between the times you see the same thing. You review a new word after a short wait, then after a longer one, then longer again. Each time you remember it, the next gap grows. When you forget it, the gap shrinks and the word comes back soon.

The result is that you spend your time on the words you are about to lose, not on the ones you already know well.

Why does spaced repetition work?

It builds on three findings from memory research that have been repeated many times since Hermann Ebbinghaus first measured forgetting in the 1880s.

• Forgetting follows a curve. Memory fades fast at first and then more slowly. Each successful review makes the curve flatter.
• Spacing beats cramming. The same number of reviews spread over days leads to longer-lasting memory than the same reviews packed into one sitting.
• Recalling beats rereading. Trying to remember a word, and checking yourself, strengthens memory more than looking at the answer again.

Flashcards combine the last two: you see the Swedish word, try to recall the meaning, and only then turn the card over. A scheduler adds the spacing.

What is FSRS?

FSRS stands for Free Spaced Repetition Scheduler. It is an open-source algorithm from the open-spaced-repetition project, built from large sets of real review data, and it is used by several flashcard apps.

FSRS describes your memory of each card with three numbers:

How does FSRS decide when to show a card again?

FSRS aims for a target chance of remembering, often set to 90 percent. After each answer it updates the card's stability and difficulty, then works out the day your chance of recalling the card will have dropped to that target. That day is when the card is due.

Your answer decides how the numbers move. A card you remember easily gains a lot of stability and comes back much later. A card you recall with effort gains less. A card you forget loses stability and returns soon, so you relearn it while it is still fresh.

One detail matters: remembering a card when its retrievability is low counts for more than remembering it when it is high. That is why reviewing a word the day after you learned it, when you would remember it anyway, does little, and reviewing it just in time does a lot.

How is FSRS different from older schedulers?

Older schedulers, such as the SM-2 algorithm from the late 1980s, multiply the last gap by a fixed ease factor after each review. They work, but they cannot tell a card you nearly forgot from one you knew well if you gave both the same answer. FSRS models how memory fades over time, so the same answer given at different moments moves the schedule by different amounts.

How does Worbi use FSRS?

Worbi schedules every card with FSRS-4.5 and a target of 90 percent recall. You do not pick a rating. You mark a card as known or not known, and Worbi derives the rating from what you did on the way:

If a word is far too easy for you, Know it well tells Worbi to push it out for months instead of days.

Smart Review

Smart Review on the Study tab is your daily session. It shows due cards first, starting with the ones that have been waiting longest, then fills up to 25 cards with new words. On a day with nothing due, it introduces new cards instead. When nothing is left to review, the Study tab shows that you are all caught up.

Mistakes

Every word you answer as not known goes on your Mistakes list, and it stays there until you get it right. Mistakes never expire. Practice mistakes starts a session of only those words, up to 25 at a time, with nothing added to pad it out.

Practicing a mistake before it is due does not move its schedule, and neither does practicing a folder or collection ahead of time. You can drill as often as you like without fooling the scheduler into thinking you know a word better than you do.

How long should a review session be?

Short and regular works best. A session of a few minutes every day keeps your due list small and lets the spacing do its work. Missing a day is fine: overdue cards simply come first next time.

Spaced repetition works for any word list. Start with the common Swedish words to learn first, and learn each noun with en or ett from the start.