Spaced Repetition

What is spaced repetition?

Spaced repetition means reviewing material not in one block but at growing intervals. Something new you look at again the next day, something well known only after one or several weeks.

The total time is often the same as cramming in one go. The difference is when you study, and that has a large effect on how long you remember.

The forgetting curve

As early as 1885, Hermann Ebbinghaus tested on himself how quickly we forget. His result, the forgetting curve: a large part of what we learn is lost in the first hours and days, after which the loss levels off. Every review makes the curve flatter, so the knowledge lasts longer.

Later research has confirmed this many times. A meta-analysis by Cepeda and colleagues (2006) covering hundreds of experiments shows that spaced learning reliably beats massed learning. A follow-up study (Cepeda et al., 2008) also found that the best gaps grow with the time until the test. The longer you want to remember something, the longer the breaks between reviews can be.

Rule of thumb from the research: if you want to know something in a month, review it days apart rather than hours apart. For an exam in a week, a shorter rhythm is enough.

The Leitner system: spaced repetition with flashcards

In 1972 the journalist Sebastian Leitner described a simple card box system that is still widely used:

1. 1

   Set up boxes

   One box with several compartments, for example five. All new cards start in box 1.

2. 2

   Correct: move forward

   When you answer a card correctly, it moves one box further back.

3. 3

   Wrong: back to the start

   A card you got wrong goes back to box 1, wherever it was.

4. 4

   Later boxes less often

   Review box 1 daily, box 2 every few days and the later boxes only rarely. That way you mostly practise what you do not know yet.

Common mistakes

• Everything the night before: cramming helps in the short term, but most of it is gone within days, which hurts in courses that build on each other.
• Too many new cards at once: new cards create many reviews over the following days. Add a small, fixed amount every day instead.
• Skipping reviews: missing a few days is no drama, but a large backlog is frustrating. Short daily sessions keep it small.
• Recognising instead of recalling: spaced repetition works best together with active recall. Answer first, then flip the card.

Sources

• Ebbinghaus, H. (1885). Über das Gedächtnis: Untersuchungen zur experimentellen Psychologie. Duncker & Humblot.
• Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380. doi:10.1037/0033-2909.132.3.354
• Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102. doi:10.1111/j.1467-9280.2008.02209.x
• Leitner, S. (1972). So lernt man lernen. Herder.