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Simply put, spaced repetition is a studying technique that lets you memorize things more efficiently and effectively. For example, let's say you want to memorize the definition of “sangfroid” for the rest of your life. There are a few possible approaches you might come up with.

1. Review the definition 20 or 30 times in a single day, until you're sure you remember it.
2. Review the definition once a week for the rest of your life, to ensure you remember it even after a few years.
3. Review the definition on a spaced repetition schedule, e.g. review it on day 1, then day 4, then day 11, then day 25, then day 60, etc. With spaced repetition, each time you successfully remember, the next review gets increasingly spaced out.

There have been a bunch of studies about the efficacy of these approaches. The basic summary is as follows. With the first approach, you'll remember what “sangfroid” means in the short-term, but will most likely forget it after a few days or weeks. The second and third approaches are similarly effective, but the third is much more efficient. Over a span of 20 years, the second approach will take up ~2 hours of your life, while the third approach will take only ~5 minutes.

There are two learning strategies that explain why spaced repetition works. The first is called active recall, and the second is called the spacing effect. Let's cover them one at a time.

Active recall is basically a fancy word for testing your knowledge. Whenever you take a test, or try to remember what some word means, you're doing active recall. The idea is that actively recalling some idea, fact, or concept helps you remember it. This contrasts with “passive review,” which is much less effective. Examples of passive review include reading a textbook or watching a documentary, assuming you're not quizzing yourself while you read or watch. Have you ever read a really interesting non-fiction book only to forget it all a few months later? That's passive review at play—if you really want to remember what you read, it's much more effective to quiz yourself on the material than to continually re-read it. For a more in-depth explanation of active recall, check out our blog post about it.

The spacing effect says that if you want to remember something, it's better to space out your reviews than to cram and do a bunch of reviews in a single day. For example, let's say you have ten reviews to spend. Instead of reviewing ten times the first day, you're better off using one the first day, then another a day later, etc. Further, there have been studies that show that a specific type of spacing is the most efficient. Specifically, instead of reviewing at fixed intervals (example 2 above), you should review at increasingly spaced out intervals (example 3 above)—the latter is just as effective as the former, but far more efficient.

“Memorization” has a bad stigma for many people—maybe it's associated with cramming for the SATs or GREs, and dissociated from more important things like learning and understanding. However, it turns out that memory plays a key role in learning any subject. This may be unintuitive at first. It makes sense that spaced repetition might be helpful for learning a language or studying for exams. But what about learning computer science, or math, or chemistry? Even for these more conceptually deep subjects, spaced repetition is still useful—spaced repetition isn't just about rote memorization. It helps you memorize key details and concepts which form the foundation for in-depth understanding and mastery. Just as it's hard to speak a language if you don't remember many words, it's hard to understand a subject if you're constantly forgetting the fundamentals.

Here are some uses of spaced repetition that we find particularly valuable:

1. Learning a language
2. Studying for exams like the SAT and GRE
3. Learning a new computer science language or API
4. Remembering the most important concepts from non-fiction books
5. Learning about a complex topic like quantum physics

If you want to learn even more about spaced repetition, here are some resources we recommend:

• Spaced Repetition for Efficient Learning
• Augmenting Long-term Memory
• How can we develop transformative tools for thought?