Spaced Repetition Explained Without the Software Sales Pitch

Almost everything you learn starts leaking out of memory within hours. That is not a personal failing, it is how forgetting works, and the pattern is predictable enough that researchers plotted it more than a century ago.

The useful part of that finding is what happens when you interrupt the leak.

The forgetting curve, and how to bend it

Memory for new material drops steeply at first, then flattens. If you review just as the memory starts to fade, the curve resets and falls more slowly the second time. Do it again and it flattens further. After four or five well-timed reviews, material can sit stable for months.

The timing matters more than the total hours. Six reviews spread over three weeks beat six reviews crammed into one evening, even though the study time is identical.

The intervals that work

You do not need an algorithm. This schedule is close enough to optimal for schoolwork.

1. Same day, a few hours after first learning.
2. Next day.
3. Three days later.
4. One week later.
5. Three weeks later.
6. Before the exam, as a final pass.

Anything you fail at a given stage drops back a step. Anything you get right easily jumps ahead.

Doing it with paper

Software is convenient but it is not the mechanism. A shoebox and index cards will do the job, and for many students the physical version is easier to sustain because it cannot be endlessly reconfigured.

• Divide a box into five sections labelled daily, every three days, weekly, fortnightly, monthly.
• Every new card starts in the daily section.
• Card answered correctly moves one section back. Card answered wrongly returns to daily.
• Study only the sections due that day.

Writing cards that are worth reviewing

Most people fail at spaced repetition because their cards are bad, not because the method is wrong.

• One idea per card. A card asking for six causes of something will always be half-remembered, which makes the scoring meaningless.
• Ask a question, do not state a topic. "Photosynthesis" is not a card. "What are the inputs and outputs of the light-dependent reactions?" is.
• Avoid copying sentences. If you can answer by recognising the shape of the text, the card is testing your eyes.
• Include a why. Cards that ask for reasons and mechanisms transfer to exam questions. Cards that ask for isolated dates rarely do.

Where spacing fits with everything else

Spacing is a scheduling principle, not a study method. It tells you when to review; retrieval practice tells you how. The two together, applied to decent cards, outperform almost anything else available to a student.

The catch is that it only works if you start early. Spacing needs weeks to operate. Two days before an exam, the only tool left is cramming, which produces a memory that survives the morning and evaporates by the weekend.

Start six weeks out with fifteen minutes a day and you will need far less in the final fortnight than the people who did nothing until then.