Spaced repetition schedule

What spaced repetition schedule can you use?

A simple schedule that suits most material is to review it 1, 3, 7, 14, 30 and 60 days after you first study it. Each gap is roughly twice the one before.

Four rules make it work:

• Make every review a test: try to recall the answer before you look (see active recall).
• If you fail an item, restart it from Review 1.
• If you miss a review, do it as soon as you can and count the next gaps from that day.
• After day 60, keep roughly doubling the gap (about 4 months, then 8) for material you want to keep.

Are these intervals backed by research?

Only partly: research shows that spaced reviews beat reviewing in one block, but the 1-3-7-14-30-60 list itself comes from practice, not from experiments that compared it with other lists. SM-2, the algorithm Piotr Woźniak described in 1990 for SuperMemo, reviews after 1 day, then 6 days, then multiplies each interval by a factor that starts at 2.5.^[1] Sebastian Leitner's 1972 card box moves cards you answer correctly into compartments you review less often.^[2]

Whether gaps should grow at all is still debated. A 2021 meta-analysis (a study that combines the results of many studies) found a strong benefit of spacing, but no clear difference between expanding gaps (such as 1, 3, 7 days) and equal gaps (such as 5, 5, 5 days).^[5] In single studies, expanding gaps kept recall higher in the short term, while equal gaps did as well or better on later tests.^[3][4] Waiting a little before the first test also helped long-term memory.^[3]

Expanding gaps are still a sensible default for a practical reason: each review covers a longer stretch of time, so items you know well take fewer reviews.

How should you space reviews before an exam?

Start early: the further away the exam, the longer the gaps between reviews can be. In a large study, Cepeda and colleagues (2008) found that the best gap before a review was about 3 days for a test a week after that review, and about 4 weeks for a test a year after it.^[6] A gap that is too long costs much less than one that is too short.^[6]

Cramming every review into the last few days leaves no room for those longer gaps. For an exam four weeks away, one reasonable plan, based on these findings rather than tested as such, is:

• Study each topic in the first week.
• Review it the next day.
• Review it again about a week later.
• Review it once more a few days before the exam.

If you need the material after the exam, keep reviewing it at growing intervals.

How do flashcard apps set intervals?

Flashcard apps replace the fixed list with a rule that sets each card's next interval from your answers. The two most common rules are SM-2 and FSRS:

• SM-2 uses 1 day, then 6 days, then multiplies the last interval by the card's ease factor. The factor starts at 2.5, rises after easy answers and falls after hard ones. A failed card starts again at 1 day.^[1]
• FSRS (Free Spaced Repetition Scheduler) estimates the probability that you can recall each card now.^[7] It schedules the next review for when that probability falls to a target you choose; Anki's default is 90%. A higher target means shorter intervals and more reviews.^[8]

Neither algorithm knows when you will forget a card. SM-2 multiplies the interval by a factor, and FSRS estimates a probability and schedules against a target.

How do you keep a schedule by hand?

The simplest way is a paper or digital calendar: on the day you study a topic, write its name on each review date from the table above.

• Calendar. At each review, test yourself and tick the topic off. If you struggle, move it to tomorrow and restart its schedule.
• Spreadsheet. One row per topic, with the first study date and each review date computed from it (+1, +3, +7 days, and so on).
• Leitner box. A card you answer correctly moves to the next compartment, and a card you fail goes back to the first. You review the first compartment daily and later ones less often.

This works well per topic or chapter. For hundreds of individual cards, an app such as Anki is easier because it tracks each card for you. See the spaced repetition apps.

How many reviews will you do each day?

Over time, expect roughly 5 to 10 times as many reviews as the new cards you add each day. It starts lower: on the schedule above, adding 20 new cards a day gives about 60 reviews a day after one week and 120 after two months, even if you recall every card.

The real number ends up higher, because failed cards restart and old cards keep coming back after day 60. The Anki manual estimates that 20 new cards a day leads to roughly 200 reviews a day.^[8] If that is more than you can keep up with, add fewer new cards.