What Is Spaced Repetition?

How spaced repetition works

Memories fade in a predictable way: quickly at first, then more slowly. Hermann Ebbinghaus first mapped this forgetting curve in the 1880s. Spaced repetition exploits it. Each time you successfully recall something just as it is starting to fade, the memory becomes more durable and the next review can wait longer.

The result is an expanding schedule. You might review a new word after one day, then three days, a week, two weeks, a month and so on. If you fail a review, the interval resets to something short and the cycle starts again.

A simple example

You can generate a schedule like this for your own words with the spaced repetition scheduler.

Why it works: the research

The underlying phenomenon is the spacing effect: the same amount of study time produces better long-term retention when it is spread out than when it is massed together. It is one of the most replicated findings in cognitive psychology. A large meta-analysis by Cepeda and colleagues (2006) reviewed hundreds of experiments and found spacing benefits across materials, ages and retention intervals. A follow-up study (Cepeda et al., 2008) showed that the ideal gap between reviews grows with how long you want to remember something.

Spaced repetition also pairs naturally with the testing effect (Roediger and Karpicke, 2006): retrieving an answer from memory strengthens it more than re-reading it. That is why most spaced repetition systems use flashcards that make you answer before revealing.

Spaced repetition algorithms

• Leitner system: a 1970s paper-flashcard method with boxes. Cards you know move to boxes reviewed less often; cards you miss go back to box one.
• SM-2: the 1987 SuperMemo algorithm, still the basis of Anki's classic scheduler. Each card has an "ease factor" that scales its next interval.
• FSRS: a modern open-source algorithm that models memory stability and retrievability from your review history, now available in Anki.

Spaced repetition for vocabulary

Vocabulary is an ideal fit, with one catch: a word learned only as a flashcard pair (ubiquitous = everywhere) often fails to transfer to real reading. Research on vocabulary acquisition stresses meeting words in varied, meaningful contexts. The strongest approach combines spaced flashcard reviews with repeated encounters in authentic text.

That is the idea behind Wordie's contextual highlighting. Every word you save is highlighted when it appears again on any site you visit, so reviews happen naturally and in context, spread out over days and weeks. A built-in recall mode then lets you test yourself directly.

Common mistakes

• Adding too many new cards. Reviews pile up quickly; 10–20 new words a day is plenty for most people.
• Skipping days. Missed reviews snowball. Short daily sessions beat long weekly ones.
• Cards without context. Include an example sentence so you learn how the word is actually used.
• Recognising instead of recalling. Say the answer before you flip the card, or the review does little.