Spaced repetition: a flashcard schedule that sticks

What is spaced repetition?

Spaced repetition is a way of scheduling review. Instead of going over a card ten times in one sitting, you see it once today, again tomorrow, again a few days later, and then at longer and longer gaps. Each review comes at the point where the answer is a little hard to pull out of memory. That slight struggle is the point: a successful recall after a delay strengthens memory far more than a recall you can do on autopilot.

Two well-studied effects sit underneath it. The spacing effect says that the same amount of study spread out over time is remembered better than the same amount crammed together. The testing effect says that trying to recall an answer is a stronger form of study than reading it again. Flashcards on a spaced schedule use both at once.

The evidence is unusually solid for education research. In a large study of more than 1,350 people learning trivia facts, Cepeda and colleagues (2008) found that the best gap between study sessions depended on how long you need to remember the material: longer test delays called for longer gaps. A 2013 review by Dunlosky and colleagues rated distributed practice and practice testing as the two highest-utility study techniques of the ten they examined.

A spaced repetition schedule you can follow

You do not need an algorithm to get most of the benefit. A fixed ladder of gaps works well for a term of coursework. This is the ladder StudyBrix uses in the Learn mode of every flashcard deck:

Notice that a card you get right four times in a row has now been retrieved across roughly four weeks (1 + 3 + 7 + 16 = 27 days). That is enough for most midterms. For a final exam or a licensing test months away, keep the card in rotation; it costs a few seconds every couple of weeks.

How the Leitner system works on paper

Sebastian Leitner, a German science journalist, described a box method for flashcards in the 1970s. It is the same idea as the table above, done with a few envelopes or a recipe box. You can run it with index cards and no screen at all.

1. Label five boxes: Every day, Every 2 days, Every 4 days, Weekly, Every 2 weeks. (The exact gaps matter less than the fact that they grow.)
2. Put every new card in the first box.
3. When a box is due, go through its cards. Answer out loud or on scrap paper before you flip.
4. A card you get right moves one box to the right. A card you miss goes all the way back to the first box.
5. Keep a small calendar note on the lid so you know which boxes are due each day.

The weakness of paper is the bookkeeping. Once you have 300 cards across three courses, an app that tracks the due date of every card saves real time. The strength of paper is that writing the card yourself is already a round of study, which is why some students make paper cards and then study them in an app.

How many flashcards should you review a day?

Your daily load is new cards plus the cards that come due. With the ladder above, each new card costs about five reviews over its first month. If you add 15 new cards a day, expect about 60 to 75 reviews a day after the first month, which takes most people 10 to 20 minutes. The number keeps creeping up after that, because older cards come back every 16 days, so add fewer new cards once reviews start taking too long.

• One course, light term: 10 new cards a day, about 10 minutes.
• Three or four courses: 15 to 20 new cards a day across all decks, about 20 minutes.
• Heavy memorization course (anatomy, organic chemistry, a language): 20 to 30 new cards a day, 25 to 35 minutes, and do not skip days.

Missing one day is not a disaster; the cards just pile up. Missing a week is what hurts, because then you face a wall of overdue cards. If that happens, stop adding new cards until you are caught up.

Fitting spaced repetition into a week

The habit matters more than the method. Attach reviews to something you already do every day, and keep new-card days separate from heavy assignment days. Here is one realistic layout for a first-year student taking four courses:

Make cards the same day as the lecture while you still understand the material. A card written two weeks later is often a card you cannot explain to yourself. For tips on wording, see how to make good flashcards.

Common mistakes that make spaced repetition stop working

Most problems come from treating the schedule as a reading list rather than a test.

• Flipping too soon. If you flip before you have committed to an answer, you are rereading, not recalling. Say or write the answer first.
• Dropping cards after one correct answer. In a 2008 study by Karpicke and Roediger, students who kept testing word pairs after first getting them right recalled about 80% a week later. Students who stopped testing those pairs recalled about a third. Both groups felt equally confident.
• Studying in small, tidy piles. Kornell (2009) had students study one big stack of 20 cards or four small stacks of 5. The big stack spaced each card further apart and worked better for about 90% of participants, yet 72% believed the small stacks had worked better after their first session.
• Cards that ask for too much. A card with a five-part answer is almost always marked wrong or marked right when it should not be. Split it.
• Using cards for things you do not yet understand. Spaced repetition keeps knowledge alive. It is a poor way to learn a concept for the first time. Work the problems or read the explanation first.

Does spaced repetition work for math and problem-solving?

Yes, with a twist. Flashcards are good for definitions, rules and the first step of a method ("What substitution would you try for the integral of 2x times cos of x squared?"). They are weaker for full multi-step problems. For courses like calculus or chemistry, space out your problem sets the same way: redo a handful of older problems each week rather than only the newest section. Our guide on studying for first-year calculus shows how.

If you want ready-made decks to practise the schedule with, the study skills deck is a short, low-stakes place to start, and the research and study skills course covers retrieval and planning in more depth.