Spaced repetition: how to schedule revision so it sticks

Spreading practice across several days produces far more durable memory than the same practice bunched into one session. It is one of the two study techniques rated as high utility in the most thorough review of the evidence, and it costs nothing: it is a scheduling decision, not extra work.

The effect

Study a topic for an hour tonight, or for twenty minutes on each of three nights. Same hour. The spaced version produces substantially better retention weeks later, and the gap widens the longer you wait to test.

The 2013 review by Dunlosky and colleagues assessed ten widely used study techniques and rated exactly two as high utility: practice testing and distributed practice. Spacing is the second.

What makes this unusual as advice is that it does not ask for more time. It asks for the same time, arranged differently.

Why cramming feels like it works

Because it does, briefly. Massed study produces strong short-term recall, so a student who crams the night before and does well has had their method confirmed. What they cannot see is the decay: most of it is gone within a fortnight.

For a one-off test of material you will never need again, cramming is a defensible choice. For anything cumulative - a final, a next course, a subject you intend to use - it is close to wasted effort, because it has to be learned again from near-scratch.

How long the gaps should be

A workable default, for something learned on day zero:

The principle underneath is to review just before you would have forgotten. Easy material can wait longer; shaky material needs shorter gaps. If a review goes badly, shorten the next one. If it is effortless, lengthen it.

Precision is not required. The difference between spacing and not spacing is large; the difference between a good schedule and a slightly different good schedule is small.

Make the review a test

Spacing tells you when. Retrieval tells you what to do when you get there, and the two multiply.

A spaced review where you reread your notes is worth something. A spaced review where you close the notes and try to recall the material first is worth considerably more, because you are getting both effects at once. More on retrieval here.

Running it without software

Apps handle scheduling well and are worth using if you like them. They are not necessary, and configuring one is a popular form of procrastination.

The paper version. A sheet per subject. When you learn a topic, write it with the date. Write the next review date beside it. When you review, tick it and write the next date. That is the entire system.

The box version. Five boxes of index cards for day one, day three, week one, fortnight, month. A card you get right moves up a box; a card you get wrong goes back to the first. This is the Leitner system, and it is a physical implementation of exactly the table above.

The weekly version. Simplest of all: one slot a week where you revisit everything from the last three weeks, from memory. Less precise, far better than nothing, and much more likely to actually happen.

Why the gaps should expand

The principle is to review just before you would have forgotten. Early on, that moment comes fast; each successful recall pushes it further out. So the intervals grow.

Getting it slightly wrong in either direction costs little. Review too early and you have spent a few minutes on something you already knew. Review too late and you have to relearn a little, which is still a retrieval attempt and still worth something. What costs a lot is not spacing at all.

Two rules that keep it honest:

• If a review goes badly, shorten the next gap. Back to a day or two.
• If a review is effortless, lengthen it. Jump straight to a month.

A worked example across one term

A student learning cellular respiration in week one of a twelve-week term:

That is 40 minutes of review spread over a term, against the two or three hours a student would spend relearning it from scratch in revision week. Spacing is cheaper in total time, not more expensive - which is the part nobody believes until they try it.

Interleaving: the companion technique

Spacing says when. Interleaving says what to mix. Practising several related topics in one session, rather than one topic to exhaustion, makes each attempt harder and the learning more durable - largely because you have to work out which method applies rather than being told by the heading at the top of the page.

A practical version: in any review slot, include questions from at least three different topics, in no helpful order. It feels worse than a block of twenty similar problems and it transfers better to an exam, where nothing tells you which chapter a question came from.

For parents

Spacing is the thing a homeschooling family can do that a classroom structurally struggles with. A class moves through a syllabus once, because thirty students cannot each have their own review schedule. At home, bringing back October's topic in November costs twenty minutes and is entirely within your control.

A simple practice: once a week, ask your child to explain something from a month ago. No notes. That single habit does more for retention than any amount of additional new content.

Where TruLearn fits

Scheduled review is one of the four things our product does. When a student gets an idea wrong, it is stored in their own words and brought back days later, as a question shaped to catch that specific gap, rather than when the student happens to think of it.

That is the part a student cannot easily do for themselves: knowing what to bring back and when requires a record of every error, and keeping that record by hand is exactly the admin that makes spacing fail in practice. How a session works, or the research.