Spaced repetition for books, not just flashcards

Spaced repetition has a reputation as a flashcard technique: vocabulary, anatomy, dates. But the idea behind it works just as well for the ideas in a book, and books are where most of us lose the most.

Why books slip away

Finishing a book feels like an ending. You close it, you shelve it, you move on. From memory's point of view, though, you've seen each idea exactly once. And things you meet once fade fast, most of it in the first few days, then more slowly after that.

That's why you can finish a book you loved and, a month later, struggle to name three things it said. Nothing was wrong with your reading. You just never came back.

What spaced repetition actually is

The idea is simple. Review something just before you're about to forget it. Each time you recall it successfully, the memory holds longer, so the next review can wait longer. Things you struggle with come back soon. Things you know well drift further and further out.

Flashcard apps made this famous, but nothing about it is specific to flashcards. It works for anything you can test yourself on, including an idea from a book.

Why flashcards alone aren't enough for books

Flashcards are great for facts: a date, a term, a formula. But most of what makes a book worth reading is an argument, a way of seeing something, a chain of reasons. That doesn't fit on the front of a card.

And if you try to make a card for every idea, a single book turns into hundreds of cards, and you quit within a week. For books, the better unit of review is the section: can you explain this part of the book in your own words?

How to do it

1. Split the book into sections. A chapter or part of a chapter, small enough to explain in a minute or two.
2. Explain each section right after reading it. Three or four sentences from memory. This is your first review.
3. Grade yourself honestly. Again: you couldn't explain it. Hard: you got there, with gaps. Good: clear, with small misses. Easy: complete and effortless.
4. Schedule the next review from the grade. Again comes back tomorrow, hard in two or three days, good in about a week, easy in two weeks or more.
5. Review from the heading only. Read the section title, explain it from memory, then check against the text and grade again.
6. Retire what you know. After a few easy reviews at long intervals, stop scheduling that section. You've got it.

A worked example

Say you're reading Seneca's On the Shortness of Life. One early section makes his central claim: life isn't short, we waste much of it.

Day 1. Right after reading, you write: "Seneca says life is long enough if you use it well. The problem is that we waste it, so it feels short." You check, and that's the core. Grade: good. Next review in about a week.

Day 8. You see only the heading. You remember the claim, but not how he backs it up: the people who spend their time on ambition, on other people's business, on endless busyness, and only notice life passing at the end. Grade: hard. Back in three days.

Day 11. This time you get the claim and his examples. Grade: good. Next review in about ten days.

Day 21. You explain it easily. Grade: easy. It can wait a month.

Four short reviews, maybe ten minutes in total, and that section of the book is now yours in a way a single read never would have made it.

How much time it takes

Less than you'd expect. Because sections spread out over time, you're only ever reviewing a handful on any given day. Ten or fifteen minutes is usually enough, even with a couple of books going at once. The early days of a new book are the busiest. After that, the queue thins out on its own.

Keeping track of the schedule

You can run this on paper: a notebook with each section, its last grade and its next date. A spreadsheet works too.

ClearRead is built around this exact loop. After you explain a section, you grade it again, hard, good or easy, and it calculates when the section comes back: "again" returns tomorrow, "easy" pushes it weeks out. When you clearly know a section, you can mark it as learned and retire it. On the Shortness of Life is one of the free library books, already split into sections, if you want to try the example above.

Start with the book you just finished

Pick the last book you read. Write down five sections you'd like to keep, explain each one from memory today, and grade yourself. Put the next dates in your calendar. In a month, compare what you remember from that book with the one before it.