How Spaced Repetition Works: SM-2, Anki and the Memory Science

Spaced repetition (SRS) is a way of reviewing so that you see each item just before you would forget it. Anki, the best-known tool, does this with an algorithm called SM-2. It is excellent for retention, and on its own it does not train speaking.

This post covers how it works and why. For using it for speaking, see Anki for speaking.

What is spaced repetition?

Memory fades, and it fades fastest right after you learn something. Hermann Ebbinghaus mapped this in the 1880s with his forgetting curve, and later research on the "spacing effect" found that spreading reviews over time beats cramming them together.

SRS software uses that. Each time you recall an item, the gap before the next review grows. Each time you fail, it shrinks. You spend time on what is slipping and little on what is solid.

Exact forgetting rates vary by material and person, so ignore any tidy percentage you see quoted.

How does Anki's SM-2 algorithm work?

SM-2 was created by Piotr Wozniak for SuperMemo in 1987. Anki's classic scheduler is a variant of it.

1. You see a card and try to recall the answer.
2. You rate it: Again, Hard, Good or Easy.
3. The algorithm sets the next interval from your rating and the card's history.

Intervals for cards you know stretch out quickly: a day, a few days, a week, a month and so on. Cards you fail drop back to short steps. Newer Anki versions also offer a different scheduler (FSRS), but the principle is the same.

What makes a good card for language learning?

• One fact per card. If you miss one item on a list card, the whole card fails.
• Sentences over single words. Quiero comer algo teaches more than comer alone, because it shows how the word behaves.
• Cloze deletions. Hide the hard part of a real sentence, such as Je vais {{c1::au}} cinéma, instead of asking for a word in isolation.
• Audio for pronunciation, when you can get it.
• Cards from your own life. Words you actually met and needed stick better than a generic list.

What are the common SRS mistakes?

• Adding too many new cards. Every card returns many times. A big daily number builds a review pile fast.
• Dishonest ratings. If you press Good when you only recognised the answer, the schedule stretches too far.
• Leaving leeches. A card you keep failing is usually badly made. Rewrite or delete it.
• Using SRS as your only method. It retains what you have learned. It doesn't replace input, grammar or conversation.

Where does SRS fall short for speaking?

Cards train recognition and recall of the thing on the card. Speaking needs you to build a sentence in real time with nobody handing you the pieces. That is why a large deck can coexist with freezing in conversation.

You can nudge it: answer out loud in a full sentence before you flip, and make cards from the mistakes you make when you actually speak. The principle behind why understanding runs ahead of speaking applies here too.

How does Cadentia use spaced repetition?

Cadentia applies the same idea to your own speech. When you talk with the tutor, your mistakes are saved and come back as review cards on a spaced schedule, and the tutor steers later conversations towards them so you have to use them again. You don't make the cards yourself.

It is not an Anki replacement or an export tool for Anki (not yet), and it is not a place to learn vocabulary from scratch.

Try a speaking check →