Spaced repetition with FSRS

At a glance

What spaced repetition is

Spaced repetition means seeing something again after a gap that gets longer each time you remember it. A card you forgot comes back soon. A card you knew comes back much later, so most of your time goes to the cards that need it.

It rests on two habits that research keeps finding help: testing yourself, and spreading your study out. In Kornell’s 2009 flashcard experiments, spacing beat cramming for 90% of the people tested, yet after their first session 72% of them believed cramming had worked better. A 2006 review of 317 experiments (Cepeda and colleagues) found that the longer you need to remember something, the longer the best gap between study sessions. A review of ten study techniques (Dunlosky and colleagues, 2013) rated practice testing and distributed practice as high utility.

What FSRS is

FSRS stands for Free Spaced Repetition Scheduler. For every card it tracks three numbers: how likely you are to remember the card right now, how many days it takes that chance to fall from 100% to 90% (its stability), and how hard the card is for you. Each answer updates them, and the next review is set for the day your chance of remembering falls to 90%.

FSRS is open source, and Anki has offered it since version 23.10. The FSRS project says it needs 20 to 30% fewer reviews than the older SM-2 scheduler for the same amount of remembering, and adds that this figure comes from simulations, not from measuring people.

Lucida uses FSRS version 5, which has 19 parameters. The newest version, FSRS-6, has 21, and Anki has offered it since version 25.07. Lucida doesn’t have it yet.

How a review works, step by step

• 1. See the front and try to answer. Say or write your answer before you flip. The retrieval practice guide from Agarwal and colleagues says that actually recalling and writing down an answer helps more than flipping the card early.
• 2. Flip and say how well you knew it. Choose Forgot, Hard, Good or Easy. Each button shows how long until you’d see the card again, like 10m, 6d, 12d or 28d.
• 3. New cards come back soon. A new card comes back after 1 minute, then after 10 minutes, and from then on it comes back after days. Press Easy on a new card and it skips the short waits. Short first waits fit Anki’s manual, which recommends learning steps under a day when you use FSRS, because repeating a card many times in one day adds little to long-term memory.
• 4. Your chance of remembering sets the gap. For a card you’ve learned, Lucida picks the whole number of days at which your chance of remembering falls to 90%, up to a year. A card you knew well gets a long gap. A card you found hard gets a shorter one.
• 5. Forgot means relearning. The card comes back after 10 minutes, then with a shorter gap than before. For a card you’ve learned, Hard, Good and Easy each give a gap at least a day longer than the button before.
• 6. Cards come due at the start of their day. Review, at the top of your Library, takes everything due: cards coming back within minutes first, then reviews, then the day’s new cards. Made a slip? Undo puts the last card back.

Missed days and backlogs

You don’t have to study every day. FSRS works from the real time since you last saw each card, so a card you still remember after a long wait counts as strong, and one you forget after a short wait counts as weak. The Anki FAQ says FSRS also handles reviews that come late, such as after a break of weeks or months, much better than Anki’s older scheduler.

Lucida has no daily review limit, so a backlog shows up in full. If it grows, lower the new cards a day for a while, because each new card brings extra reviews later. Anki’s manual estimates that learning 20 new cards a day settles at roughly 200 reviews a day, and says that adding new cards while you’re behind can make a backlog worse.

Pick your remember goal

The remember goal is the chance you want of remembering a card when it comes back. A higher goal means shorter gaps and more reviews. Anki’s manual says 90% is a good balance, that the workload climbs quickly above 90%, and that above 97% it can be overwhelming. Lucida uses 90% for every deck, so there’s nothing to set.

Cards you keep forgetting

After you’ve forgotten a card 8 times, Lucida tags it Leech so you can find it. Anki calls these cards leeches too. Its manual says the most efficient fix is to change how the information is presented, since often the card holds too much or you don’t fully understand it yet.

On Pro you can change the count (from 3 to 30) and choose to pause leeches instead of only tagging them. Pro’s Stats page lists them, with a button to study just those cards.

Exam dates (Pro)

Add an exam date to a deck. From 30 days before, a card comes up early once your chance of remembering it slips under a target that rises from 90% to 95% on exam day. So the cards you’d probably forget by then get an extra look, and the whole deck is fresh for the test. Each of those reviews is a normal one, so after the exam your usual schedule carries on.

Stats shows, for each exam deck, the days left, how many cards you’ve seen and learned, and how much of the deck you’d likely remember on the day if you stopped studying now.

Tune to you (Pro)

FSRS starts from standard parameters from the FSRS project. After 400 graded reviews (at least 100 of them reviews of cards you’d already learned), Pro can fit FSRS’s 19 parameters to your own history, so the gaps match how fast you really forget. It runs in your browser, in the background. When you’ve added a quarter more reviews, it quietly tunes again, and you can switch back to the standard settings at any time.

Anki’s manual says the same kind of optimizer has little to learn from when there are fewer than a few hundred reviews.

What Lucida doesn’t do

• It doesn’t use FSRS-6 yet. Lucida runs FSRS-5.
• It doesn’t read your Anki review history. Cards you import start as new cards (see importing).
• Gaps are exact, with no random spread, so cards you learned together tend to come back together.
• It doesn’t work offline. Lucida needs an internet connection.
• The 20 to 30% figure above is the FSRS project’s own simulation result, not a measurement of Lucida’s users.

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