Spaced repetition: what it is and how to study with it
Spaced repetition means reviewing the same material several times, further apart each time. Instead of studying a topic for four hours straight on a Sunday, you look at it today, tomorrow, in three days, in a week and in two weeks. The total time is about the same, sometimes less, but what's still in your head on exam day is a lot more.
The reason is that we forget quickly and predictably: most of what you study fades in the first few days, and every review makes what's left last longer. Spaced repetition takes advantage of that. You don't review when you feel guilty, or all at once the night before: you review right as you were starting to forget.
Here's how I use it as a final-year computer engineering student, with a real example: Computer Networks, a course where you have to keep layers, protocols, ports and headers in your head, and where cramming the week before never got me far enough.
What spaced repetition is
It's a study technique with two parts:
- Review more than once. Something you saw only once is almost completely gone within a few weeks, even if you understood it perfectly.
- Space the reviews out, more and more. The first review comes soon, the next day. The later ones drift apart: a few days, a week, two weeks.
The opposite is what most of us do: cramming. Studying everything in one long session right before the exam. Cramming can get you through tomorrow's test, sometimes, but what goes in that way leaves just as fast. And in a degree where every course builds on the last one, you pay for it next semester.
Why it works: the forgetting curve
In the late nineteenth century, Hermann Ebbinghaus spent years memorizing lists and measuring how much he remembered over time. What he found is known as the forgetting curve: forgetting is steep at first and then flattens out. We lose a lot in the first hours and days, and whatever survives that fades more slowly.
The interesting part is what happens when you review. Each review lifts the memory back up, and the curve that follows is gentler than the one before: you take longer to forget. That's why reviews can be spaced further and further apart. Early on, the memory is fragile and needs a review soon; later, it holds up on its own for longer.
This is called the spacing effect, and along with active recall it's one of the most studied findings about how we learn. There's one more twist that feels backwards: a review is harder once you've started to forget, and that's good. The effort of rebuilding something that was slipping away is what makes it stick. Reviewing something that's still fresh feels great, but does very little.
You don't review so you won't forget. You let it start to slip, then pull it back: that's what makes it stay.
How to build a review schedule
You don't need an exact formula. What matters is the shape: reviews that start close together and spread out. This is the one I use:
| Review | When |
|---|---|
| First | The day after the lecture |
| Second | Three days later |
| Third | One week later |
| Fourth | Two weeks later |
| Fifth | A month later, or exam week |
In Networks, the transport layer lecture was on a Tuesday. On Wednesday I did the first review (ten minutes), on Friday the second, the following Tuesday the third, and so on. Each one was shorter than the last, because there was less to rebuild every time.
Two rules make the schedule actually work:
- What you miss comes back sooner. If at the one-week review you couldn't remember how TCP flow control works, that concept doesn't wait two weeks: it comes back in a couple of days.
- What you nail can wait longer. If it comes out on its own, you don't need to see it every three days. That time belongs to what keeps slipping.
A simple way to track it without any app: a sheet with topics in rows and review dates next to them. Cross one off when you review it and write the next date, closer or further depending on how it went.
Every review has to be recall
This is the most common mistake. Spacing your reviews doesn't do much if every review is rereading the slides. Every review should start with your notes closed: you try to say or write what you know, and only then do you look.
For the transport layer, my reviews went like this:
- I read only the topic name, for example "TCP: connection setup".
- I write down what I remember: the three-way handshake, what each segment carries, why it's needed at all.
- I open the slides and compare. Whatever wasn't on my sheet is what comes back sooner.
That's active recall, and spaced repetition is basically active recall spread out over time. I went into it in detail in active recall, and into why rereading fools you in does rereading work.
Flashcards, apps and what I use
The best-known form of spaced repetition is flashcards: a question on one side, the answer on the other. The Leitner system does it with boxes: a card you get right moves to the next box, which you review less often, and a card you miss goes back to the first one. Apps like Anki do the same thing and work out on their own when each card is due.
Flashcards work great for pure memory: ports, acronyms, dates, vocabulary, formulas. They work less well for understanding a whole process, because they force you to chop the topic into tiny pieces, and making them takes time. Time that, in a week with three midterms, you often don't have.
That's why, when I built Filmina, the web app that explains my lecture slides out loud, I put spaced repetition inside Practice: what you missed comes back the next day, what you got right once comes back in three days, and what you got right twice in a row, in a week. The questions come from the lecture you studied, so there are no cards to write. It's all in practice and review.
App or no app, the point is the same: let what you remember and what you don't decide the schedule, not how motivated you feel that day.
Common mistakes
- Starting in exam week. Spaced repetition needs time to space things out. If you start five days before, it still helps, just with reviews closer together. Ideally you start with the first lecture.
- Reviewing everything every time. If every review covers the whole topic, in three weeks your day won't be long enough. Review what you missed and let what you know rest.
- Skipping the first review. It's the most important one, because it lands when forgetting is steepest. Ten minutes the next day is worth more than an hour the following week.
- Making cards with one-word answers. Mix questions about what, why and how. "Which port does HTTPS use?" is fine; "why does TCP need a three-way handshake?" is better.
Frequently asked questions
What is spaced repetition?
It's a study technique where you review material several times, with longer gaps between each review. It's based on the forgetting curve: every review makes what you learned last longer before it fades.
How often should you review with spaced repetition?
There's no single interval that works for everything, but a common schedule is the next day, three days later, a week later and two weeks later. What you struggle to remember comes back sooner, and what you already know can wait longer.
Can you do spaced repetition without flashcards?
Yes. A list of topics with review dates next to them is enough. What matters is that each review starts with trying to remember without looking, whether that's writing on a blank page or explaining it out loud.
Does spaced repetition work if the exam is in a few days?
It does, with reviews closer together: the same day, the next day and the day before the exam. It beats one long cramming session, although the biggest payoff comes when you have weeks ahead of you.