How Spaced Repetition Works (and Why Cramming Fails)

Here is a familiar pattern. You sit down, review forty new words and feel good about it. Two days later, half of them look unfamiliar again. That is not a flaw in your memory. It is how memory works for almost everyone, and it is the problem that spaced repetition is designed to solve.

Why new words fade

A new memory starts out fragile. The standard picture in memory research is that people forget newly learned material quickly at first and then more slowly. The details vary with the material and the person, but the pattern is steady: the first hours and days are when most of the loss happens.

Cramming makes this worse in a quiet way. Right after you see a word, recognizing it is easy, so a long session feels productive. But recognizing a word a minute after you studied it says little about whether you will recall it next week. The feeling of knowing and the fact of knowing come apart, and that is why cramming so often disappoints.

What spacing means

Spacing means splitting your study into separate sessions with gaps between them, instead of packing everything into one. Reviewing a word at growing intervals, a day later, then a few days later, then a week or more, is called spaced repetition. Each successful recall makes the memory more durable, so the next review can wait a little longer.

The idea is not new, and it is one of the better-supported findings in the study of learning. One large review of the research on distributed practice in tasks that involve remembering words and other verbal material combined 839 assessments from 317 experiments in 184 articles. It found that the spacing between study sessions and the time until the final test work together: the gap that produces the best memory gets longer when you need to remember for longer.

How long should the gaps be?

The most useful answer comes from a study that tested gaps over long periods. More than 1,350 people learned a set of facts, then reviewed them after a gap of up to three and a half months, and were tested again as late as a year after the review. At any test delay, lengthening the gap between study sessions first improved the final score and then, past a point, lowered it. The best gap grew as the test delay grew.

When the researchers expressed the best gap as a share of the time until the test, it shrank as the test moved further away. For a test about a week away, the best gap was roughly 20 to 40 percent of that week. For a test about a year away, it was roughly 5 to 10 percent of the year. That is a rule of thumb you can use: the further away the moment you need to remember, the longer the gap between reviews can be, but it does not grow as fast as the delay.

Two cautions apply. The facts in that study were not vocabulary, though the broader review covers word-based tasks. And the numbers describe averages across many people, not a schedule that is exact for you.

What a study of language learners found

A study with 128 Japanese college students looked at spacing in English vocabulary learning. Each student studied 20 English-Japanese word pairs under different schedules, either with equal gaps or with gaps that expanded, and with different amounts of spacing from none to long. The results showed a limited but statistically significant advantage for expanding gaps, and the effect of spacing at all was large. The authors' summary was that expanding gaps may help, but that introducing any spacing may matter more than which schedule you pick.

That is reassuring. You do not need a perfect algorithm. If your choice is between one long session and several shorter ones across the week, choose the several.

Spacing works better with testing

Spacing pairs well with trying to recall the word before checking it. In a well-known set of experiments, students who took recall tests on a passage remembered more after two days and after a week than students who simply reread it, even though the rereaders felt more confident. Only on a test five minutes later did rereading win. The practical point is that each review should ask you to bring the word to mind, not just to look at it again. Active recall versus rereading goes into this in more detail.

A simple schedule you can follow by hand

If you keep your own list, you can space your practice with nothing more than a calendar. Learn a new word today. Test yourself tomorrow. Test again three or four days later. Test again a week after that, then after two weeks, and then after a month. If you miss a word at any step, move it back to the next day's list. The exact numbers are an example, not a rule from the research, and you should adjust them to how fast you forget.

One more practical point concerns new words per day. It is tempting to add as many as possible, and the reviews pile up fast. Each new word creates a series of reviews over the next weeks, so twenty new words a day soon means an hour of reviewing. A smaller number you actually review is better than a larger number you abandon. Five to ten a day is a reasonable range for most people.

Common mistakes

• Reviewing too soon. If you check a word again five minutes later, you are practicing the feeling of recognition, not recall.
• Waiting too long. If a word has gone completely, you are relearning it. A gap that is a little uncomfortable, where you have to struggle slightly, is usually about right.
• Marking words as known when you only half know them. An honest answer decides when the word returns.
• Spacing everything the same way. Hard words need shorter gaps and easy words longer ones.

How Wordabulary applies the idea

In Wordabulary's practice mode, you choose between Got it and Still learning after each word. A word you mark Got it is shown less often, and one you mark Still learning comes back sooner, so your answers decide the gaps. A weekly email can remind you when words are ready for review. Honest answers matter, because the schedule is only as good as the information it gets from you.