Spaced Repetition Explained

Spacing is the part of revision students most often skip, because it means stopping while material still feels fresh. That is exactly when a review does the most work. The method is old, well tested, and cheap to run once you know the timing.

Why spacing works

Memory fades on a curve. Hermann Ebbinghaus mapped it in the 1880s and found that much of what he memorised vanished within days unless he revisited it. Each successful recall after a gap flattens the curve, so the next forgetting takes longer.

The mechanism that matters is effort. Recalling something just before you would have forgotten it is harder than re-reading, and that difficulty is what rebuilds the trace. Retrieval and spacing work as a pair: one is the action, the other is the timing.

This is why cramming feels effective. Massed repetition keeps material in mind for tomorrow's exam and leaves very little a month later. Spacing trades a worse feeling now for retention that lasts past the end of the course.

• Forgetting curve - The pattern by which recall drops over time without revisiting the material.
• Retrieval - Pulling information from memory rather than reading it again on the page.

Building a schedule that fits a term

Review new material the same day, again after three days, then after a week, then after a month. That widening pattern covers a term's content in a handful of short sessions per topic instead of one long revision block at the end.

Space by difficulty. Names, formulas and vocabulary need more reviews than a concept you can already explain aloud. A prompt you keep failing is signalling an interval that is too long, not a memory that is broken.

Put the reviews in a calendar rather than in your head. Vague intentions happen the night before the exam. Twenty minutes at the same time each weekday keeps a year of course material alive.

Protect the first review. The same-day pass is the cheapest of the set and the one students skip, because the material still feels fresh and the session feels unnecessary. That freshness is exactly why it is short: five minutes today saves thirty minutes a fortnight later.

• Expanding interval - Reviews spaced one day, three days, a week, then a month apart.
• Lapse - A review you fail; shorten the gap and relearn the item from your notes.

Prompts, cards and questions

An item is only as good as its prompt. What is diffusion is a weak card, because the answer is a definition. Why does a gas diffuse faster when it is warm is better, because answering it needs understanding rather than recognition.

Cloze deletions suit sequences and formulas: remove the key term from a sentence and rebuild it from memory. Keep each card to one fact, because a card holding three facts gets failed for one of them and loses the other two as well.

Write the cards from your own notes rather than downloading a shared deck. Making the card is a review in itself, and the structure of your course is not the structure of someone else's. Keep the source reference on the back of every card, so a prompt you keep failing sends you back to the section rather than to a guess.

• Cloze deletion - A prompt made by removing a key term from a sentence and recalling it.
• Prompt quality - A question that can be answered only by understanding, not by recognising.

Do you need the software

Anki and similar programs run a scheduling algorithm, based on Piotr Wozniak's work on SuperMemo in the late 1980s, which sets each interval from how easily you recalled the item. These tools work well and none of them is required.

A paper list runs on the same principle: date each item and revise anything not seen in three days. What the software adds is scheduling discipline, and a checklist plus a calendar reproduces that well enough for a degree.

Whichever you pick, the algorithm is not the active ingredient. Recalling the answer is. Hours spent tuning settings are hours not spent reviewing.

• Scheduling algorithm - A rule that sets each interval from how easily you recalled the item.

Common mistakes

Making cards instead of using them is the first. A deck of a thousand reviewed twice is worse than a deck of two hundred reviewed twenty times.

Treating recognition as recall is the second. If the answer looks familiar when you turn the card and you move on, you practised recognition rather than recall. Say or write the answer before you check.

Abandoning the system after a missed week is the third. A lapse means the queue is longer, not that the method failed. Cut the deck down to what carries marks and start again from today.

Reviewing is not the same as revising, and the difference matters. A review keeps material available; it does not build understanding of something you never grasped. When a prompt keeps failing, go back to the explanation with a tutor or a textbook before adding more reviews.

• Recognition - Knowing an answer when you see it, which is weaker than producing it.