Spaced Repetition: Scheduling Yourself to Remember | Memory Techniques Tutorial

This chapter covers the forgetting curve, Leitner boxes, and how software like Anki makes review scheduling automatic.

Ebbinghaus and the Forgetting Curve

In 1885, Hermann Ebbinghaus, a German psychologist, spent years memorizing lists of nonsense syllables and testing when he forgot them. His graphs became the forgetting curve.

The shape, verified many times since:

Retention
  100% ─┐
        │\
        │ \
   50%  │  ────────
        │          ────────
        │                   ────────
    0% ─└───────────────────────────────── Time
       0   20min   1hr   1day   1wk   1mo

You lose most of what you learned in the first hour, and by the next day you've forgotten half. Without review, a week later, you remember maybe 20% of what you learned.

The curve isn't fate. Each review flattens it. After one well-timed review, the curve decays more slowly. After several reviews, material sticks for months or years.

The key phrase is "well-timed".

The Spacing Effect

The same amount of study, spread over time, beats the same amount in one block. If you have an hour to study X:

60 minutes in one day: mediocre retention in a week.
20 minutes a day for 3 days: much better retention in a week.
10 minutes a day for 6 days: better still.

The brain consolidates during the gaps. Cram sessions feel productive; they produce fragile memories.

Space your study. If you're learning something important, don't plan one three-hour session. Plan six thirty-minute sessions.

The Leitner Box

Physical spaced repetition. Invented by Sebastian Leitner in the 1970s.

Five boxes. New cards start in Box 1.

Box 1    reviewed daily
Box 2    reviewed every 3 days
Box 3    reviewed every week
Box 4    reviewed every 2 weeks
Box 5    reviewed monthly

When you review a card and get it right, move it up a box. Get it wrong, send it back to Box 1.

Over time, mastered cards end up in Box 5, reviewed rarely. Struggling cards stay in Box 1, reviewed daily. The box you work in each day is much smaller than the full deck, because most of it is in Boxes 3 to 5.

The Leitner box is low-tech, hand-operated, and works. If you prefer paper, use it. It also helps you understand what Anki is doing.

SuperMemo and SM-2

Piotr Wozniak, a Polish computer scientist, noticed in the late 1980s that review intervals could be tuned per card based on how well you remembered it. In 1990 he published the SM-2 algorithm, which adjusts each card's next review based on your own grading of how easy recall was.

The algorithm in one paragraph: each card has an ease factor. After you review it, you rate your recall (from 0 = total failure to 5 = trivial). Cards rated 4+ get their interval multiplied by the ease factor. Cards rated 3 keep the interval. Cards rated below 3 reset to a short interval. The ease factor itself adjusts up or down based on your ratings.

The details don't matter for using it. What matters: SM-2 is what Anki uses (with modifications), and it gives you the best interval for each card based on your actual history.

Anki: The Practical Tool

Anki is free, open-source software that implements spaced repetition for flashcards. It runs on desktops, phones, and the web. Millions of students use it. Medical students, language learners, and competitive memory people all swear by it.

The Basics

You create decks. A deck is a collection of cards.
Each card has a front and a back.
Every day, Anki shows you a mix of new cards and cards due for review.
For each card, you rate how well you recalled it: Again (failed), Hard, Good, Easy.
Anki schedules the next review accordingly.

Over time, the workload stabilizes: you review cards that are due, not the whole deck.

A Realistic Daily Session

New cards:       10 per day (you can tune this)
Reviews:         starts small, grows to ~100-200/day over months for active learners
Session time:   15-25 minutes, most days

Sessions take about 15 minutes once you've been at it for a while. Less for light users; more if you're serious. The pattern is: short daily sessions, forever.

Getting Started

Install Anki from ankiweb.net. Pick a topic. Make 20 cards. Review daily.

That's it. The software handles the math.

Card Design for SRS

Chapter 5 gave the general rules. A few things specific to SRS.

Minimum Information Per Card

SRS shows each card independently. A card with five facts on it is five chances to fail. Break it up.

Don't Import Huge Decks

You can download pre-made decks (Anki has thousands). They're tempting. They're usually bad for you because someone else made them; you didn't engage with the material to create the cards.

Cards you wrote are better than cards you downloaded. The writing is where the understanding forms.

Reverse Cards

For some material (vocabulary), it's useful to have both directions: English → Spanish, and Spanish → English. Anki can auto-generate reverse cards. Use them when both directions matter.

Edit Ruthlessly

If a card keeps tripping you up, rewrite it. If it's ambiguous, clarify. Anki is a skill too; your cards get better with experience.

Review Discipline

SRS works only if you review daily. Miss a week and you'll have a giant pile of backlog that takes multiple hour-long sessions to clear.

Rules that help:

Review every day, even 5 minutes. Keeping the streak is part of the habit.
If you miss a day, don't quit. Clear the backlog over a few days.
Don't try to "finish" a deck in one sitting. It's a habit, not a project.

Most people who "tried Anki and it didn't work" stopped reviewing for a week, saw the backlog, and quit. Miss as few days as possible. Treat it like brushing teeth.

When SRS Doesn't Help

SRS is optimized for facts that are discrete, testable, and have clear answers. It's not magic.

Less suited:

Skills that require doing, not knowing. Programming, music, sports. Practice the skill, not flashcards about it.
Deep conceptual material. Flashcards over differential equations don't replace working problems.
Short-term needs. Cramming for tomorrow's exam is not what SRS is for. SRS builds over months.

Best for: vocabulary, factual material, names and dates, procedural steps you need to recall on demand, anything with a large corpus you'll use for years.

A Study Loop That Works

Combine active recall and spaced repetition:

Learn the material (read, understand, work an example).
Write flashcards for the atomic facts and concepts you want to retain.
Review daily using Anki (or a Leitner box if you prefer paper).
When a card fails, investigate: is the card badly written, or is the knowledge weak? Fix the appropriate thing.
Add new cards gradually as you learn more.

This loop, applied consistently over months, has carried medical students through board exams, language learners to fluency, and memorizers to championships. It works because the two pieces (recall and spacing) are each well-supported by research, and together they compound.

Common Pitfalls

Downloading a 10,000-card deck and expecting to learn. You haven't engaged with the material. Make your own.

Cramming new cards. "I'll add 200 new cards today." In two weeks you'll have 800 reviews/day and quit. Start with 10 a day, raise gradually.

Reviewing inconsistently. A week off generates a backlog that feels insurmountable. Daily is the discipline.

Treating Anki as the only tool. SRS is one tool among many. Combine with active recall on full passages, practice problems, and palaces for ordered material.

Not deleting bad cards. Some cards turn out to be wrong, redundant, or badly worded. Delete or rewrite them instead of powering through.

Next Steps

Continue to 07-numbers.md to start remembering digits at scale.