Spaced Repetition

6 minute read

In today’s fast-paced, information-driven world, everyone must constantly learn new things. While learning itself can be challenging at times, remembering the information after you’ve learned it over months or years is far more challenging. Believe it or not though, there’s a study method backed by a heap of scientific evidence and a large number of regular users that can virtually eliminate this difficulty, so long as you’re able to put in a small amount of daily effort.

The key to long-term retention is the spacing effect. Study after study has found that rehearsing material in a number of widely-separated study sessions is far more effective than clumping the rehearsal together. Some studies report spacing alone can improve learning efficiency by 50% or more!

Combining the spacing effect, active recall techniques (described below), and intelligent separation of easy things from hard things yields the learning technique called spaced repetition. Spaced repetition makes it straightforward to remember large amounts of material (fifteen thousand flashcards, added gradually over time, is a walk in the park) for months or years, even if you do nothing with the material beyond reviewing it for a few minutes a day. It does this by optimizing your study sessions so that you review things as close as possible to the time that you’re about to forget them – earlier is a waste and later means you have to relearn the item.

Tip: Trying to learn a language, study for a large exam, or pick up a large discipline like medicine or law without using spaced repetition is as silly as doing all the reading with your reading glasses sitting on the table beside you. If you don’t take advantage of spaced repetition, you are throwing your time away!

In the remainder of this post, we’ll take a quick look at each of those three components: spacing, active recall, and separation of difficulty. In the next post, we’ll explore how spaced repetition scheduling works, and in the post after that, we’ll explore how you can get started.

The spacing effect

You can get a great deal from rehearsal
If it just has the proper dispersal.
      You would just be an ass
      To do it en masse
Your remembering would turn out much worsal.
Ulric Neisser

As mentioned earlier, the spacing effect states that rehearsing material in a number of widely-separated study sessions is far more effective than clumping the rehearsal together. On the one hand, this is utterly intuitive. It makes sense that cramming for four hours and then not touching your material for a month is less effective than studying for an hour a week. But on the other hand, studying using the spacing effect feels harder and less effective, making it rather unintuitive and perhaps explaining why such a valuable technique is still unpopular. Most typical study methods result in spending a large proportion of study time rehearsing things you have already rehearsed recently, often followed by not studying those things for so long that you forget them again. Obviously, studying things you already know and will soon forget without review is a poor use of time, but it seems like you’re rolling right along doing a great job. In contrast, when you wait to study things until you’ve had some time to forget them, you must work harder to recall them, and you spend a much larger proportion of time working on the hard stuff, so it feels like you’re doing poorly and having trouble! However, if you stick with it, you’ll soon find it results in clear long-term benefits. (This phenomenon is sometimes called desirable difficulty.)

The exact intervals between study repetitions that yield the best results are a matter of much debate, and experimental results are mixed. However, we will look at several popular algorithms that do a perfectly acceptable job, whether they are close to optimal or not. Most important is that you use spacing of any kind at all.

Note: The spacing effect is less valuable over short learning periods. If you have only an hour or a day to learn something, cram away using whatever techniques you like. But if you need to retain the information for more than a week or so, it’s time to start using spacing.

Active recall

The testing effect states that using information improves memory more than encountering it does. For instance, in one recent study, students studied a text, then immediately took one to three tests on it or restudied the text one to three times (the exact number of times varying depending on the condition people were assigned to). A week later they took a final test. Students who took the tests as opposed to restudying the text performed as much as a third better on the final test – even though the studiers felt more confident. This effect doesn’t even require feedback – the students who took tests as practice never even saw their scores on those tests, much less which questions they got wrong.

The easiest way to incorporate active recall into learning, and the method used by virtually every spaced-repetition tool, is using flashcards, whether physical or virtual – look at the question, test yourself on the answer, and get immediate feedback. If practice tests are readily available for the material you’re studying, those can be valuable, but flashcards allow breaking the questions down into small units that are more convenient to work with on a regular basis. Plus, it’s straightforward to create flashcards for anything, even information nobody has ever learned before.

Separating easy things from hard things

This section’s title sounds like a dumb revelation, when you look at it – doesn’t it? Surely, when you already know something, you’re going to stop studying it. But oddly, people often don’t do this, perhaps again because it’s much easier to keep studying easy things. Further, many study methods don’t make this easy. For instance, if you’re using an online learning tool or taking practice tests supplied by someone else, you may or may not be able to control when material appears for you to review.

However, this separation is critical to effective learning. It’s not uncommon for particularly difficult information to need review two to three times as often as easy information. That means if you don’t sort it out, you may have to study your easy material as much as three times more often than necessary, just to be successful at retaining the hard information!

The obvious problem with spacing out reviews further is that it’s more likely you forget a given piece of information before you study it again. To reduce the risk, spaced repetition tools have algorithmic means to adjust when information should be reviewed next based on how hard it is. For instance, you might look at something you completely forgot in just ten minutes, whereas something you’ve reviewed many times before and still easily remembered could go several weeks. We’ll look at ways to compute these times and manage reviewing next week.