In short
- Spreading reviews of a word over days and weeks gives better long-term memory than packing the same reviews together, in second language learning as elsewhere.
- The best gap depends on how long pupils must remember: in one large study it was 20 to 40% of a one-week delay, but only 5 to 10% of a one-year delay.
- Gaps that grow longer each time show no clear advantage over equal gaps; having real gaps, some of them long, matters more.
- You can run spacing on paper with a word bank kept by week and a five-minute review that reaches back.
Most vocabulary review in school happens at one of two times: straight after the words are taught, or the night before a test. Both pack the practice together. Spaced repetition spreads it out, so each word comes back after a gap. It is one of the best-supported ideas in memory research. This article explains what spacing is and what the main studies found about how long the gaps should be. It asks whether the gaps should grow, and shows how to build spacing into a class week with a list and a calendar.
What is spaced repetition?
Spaced repetition means meeting and recalling the same word several times, with gaps of days or weeks between the meetings. The opposite is massed practice: the same number of repetitions in one block. For vocabulary, a spaced schedule might bring a word back the next day, a week later and a month later.
Three terms help when you read the research:
- The gap is the time between two study sessions. Researchers also call it the interstudy interval.
- The test delay is the time between the last study session and the test. Researchers call it the retention interval.
- Expanding spacing makes each gap longer than the last. Equal spacing keeps the gaps the same.
Why spacing works is still debated. Cepeda and colleagues (2008) argued that their own data fitted some explanations better than others. The effect is better established than its cause, and you can use it without settling the theory. How many meetings a word needs in total is a separate question, covered in how many times must a learner meet a word?
What the research found
Cepeda, Pashler, Vul, Wixted and Rohrer (2006) reviewed 839 assessments of distributed practice from 317 experiments in 184 articles on verbal recall. Spacing helped retention. The gap that gave the best retention grew as the test delay grew.
Kim and Webb (2022) asked the same question for second language learning. They combined 98 effect sizes from 48 experiments, with 3,411 learners. Spacing had a medium-to-large effect. Shorter gaps did as well as longer gaps on immediate tests, but worse on delayed tests.
Dunlosky and colleagues (2013) reviewed ten study techniques. Distributed practice was one of only two they rated high utility. Its benefits held for learners of different ages and abilities, and had been shown in real classrooms.
| Study | Learners and material | Finding |
|---|---|---|
| Cepeda et al. (2006) | Review of 317 experiments on verbal recall | Spacing helped; the best gap grew with the test delay |
| Cepeda et al. (2008) | 1,354 adults online; 32 trivia facts | The best gap was a shrinking share of the test delay; too short cost more than too long |
| Kornell (2009) | Participants learning word pairs with a web-based flashcard program | One large stack beat small stacks for 90% of participants, though most believed the opposite |
| Karpicke & Bauernschmidt (2011) | 96 university students in the United States; Swahili-English word pairs | More total spacing gave large gains; expanding, equal and contracting schedules did not differ |
| Nakata (2015) | 128 Japanese college students; 20 English words with translations | A small advantage for expanding gaps; the amount of spacing mattered more |
| Kim & Webb (2022) | Meta-analysis of 48 second language experiments, 3,411 learners | Medium-to-large effect; longer gaps better on delayed tests; equal and expanding equivalent |
One caution runs through the table. Most of the single studies used university students or other adults, and the material was word pairs or facts, not words met in stories.
How long should the gap be?
Cepeda, Vul, Rohrer, Wixted and Pashler (2008) ran a large test of this question. In an online study, 1,354 adults learned 32 obscure trivia facts. They reviewed the facts after a gap of up to 105 days, then took a final test 7, 35, 70 or 350 days later.
For each test delay, recall first rose as the gap grew, then slowly fell. The best gap got longer as the test delay got longer. As a share of the test delay, though, it got smaller. Compared with no gap at all, the best gap raised final recall by 64%.
| Test delay | Best gap (approx.) | Gap as a share of the delay |
|---|---|---|
| 7 days | 3 days | 43% |
| 35 days | 8 days | 23% |
| 70 days | 12 days | 17% |
| 350 days | 27 days | 8% |
Two practical lessons follow. First, decide how long the words must last before you set the gaps. Words for a quiz next week need a gap of days. Words for the end of the year need gaps of weeks. Second, err on the long side. The authors found that a gap longer than the best one cost much less than a gap that was too short.
Treat the numbers as a rough guide. The material was trivia facts, not words, the learners were adults, and there were only two study sessions.
Should the gaps get longer each time?
Nakata (2015) notes that expanding spacing is often regarded as the most effective schedule, but that earlier comparisons gave mixed results. In his own study, 128 Japanese college students learned 20 English words with their Japanese translations. Expanding gaps gave a limited but statistically significant advantage over equal gaps. The amount of spacing, from none to long, had a larger effect.
Karpicke and Bauernschmidt (2011) had 96 students learn Swahili-English word pairs until they could recall each one. Each pair was then recalled three more times. Long gaps between those recalls produced a 200% improvement on a test a week later, compared with no gaps. Whether the gaps expanded, stayed equal or shrank made no detectable difference. Here the gaps were measured in study trials within one session, not in days.
Kim and Webb's (2022) meta-analysis found equal and expanding spacing statistically equivalent. The message is reassuring. Do not worry about the exact pattern. Make sure each word comes back several times, and that some gaps are long. Expanding gaps are still convenient, because they fit a calendar: next lesson, next week, next month.
How to build spacing into a class week without software
You need a word bank, a fixed review slot and a record. The system below runs on a notebook or a shared document. It is a sketch to adapt, not a tested program.
- Keep a word bank by week. Each week, write the six to ten key words from the class reading under the week number.
- Open every lesson with a five-minute review. Pupils recall words without looking, then check.
- Draw from three lists each time. Take words from the last lesson, from one week ago and from about four weeks ago.
- Make every review a recall. Give the meaning and ask for the word, or gap a sentence from the story. Do not just read the list aloud.
- Mark the slips. A word most of the class missed goes into next week's list as well.
- Retire words slowly. Take a word out of review only after the class has recalled it after a gap of several weeks.
Classroom example (a planning sketch, not a tested schedule)
Week 1, Monday: the class reads a chapter with the words cliff, gull, shelter, gale, drift and anchor.
Week 1, Tuesday: the teacher reads gapped sentences from the chapter, and pupils write the missing word.
Week 2, Tuesday: the same words return with picture prompts, mixed with this week's words.
Week 5: they return a third time, mixed with words from weeks 4 and 5. Half the class misses gale and drift, so both go into week 6 as well.
Week 10, before the end-of-unit quiz: a mixed recall of words from weeks 1 to 9.
The five-minute review can be a short daily quiz, a round of word cards or a game. A paper system has one real limit. It schedules words for the class, not for each pupil, as if everyone forgot at the same rate. Software can schedule per pupil. A paper list run every lesson still keeps what matters most: recall, spread out, with some long gaps.
Common mistakes with spacing
- Reviewing too soon. Going over words again later in the same lesson is still massed practice. Kim and Webb (2022) found that short gaps matched long ones on immediate tests and fell behind on delayed tests.
- Cramming before the test. Cepeda and colleagues (2008) warned that packing study into a short period gives misleadingly high immediate mastery that does not last.
- Stopping after the first correct answer. In Karpicke and Bauernschmidt (2011), how far apart the later recalls came made a large difference a week later.
- Trusting how it feels. Kornell (2009) found that after the first session, 72% of participants believed massing had worked better. Spacing won for 90% of them. Pupils will often prefer the cram.
- Spacing only the words. In practice, the same idea works for a story's plot or moral. A two-minute recall of last month's story is a spaced review too.
Spacing is one method among several. How to help students memorize English vocabulary compares it with word cards, reading and the keyword method.
What the research does not tell us
The evidence for spacing is strong, but it has gaps for school use. Kim and Webb (2022) found that the size of the spacing effect varied with the learning target, the number of sessions, the type of practice and activity, feedback timing and the retention interval. We found no study that sets a best schedule for pupils aged 8 to 16 learning words from stories.
So run your schedule as a trial. Every few weeks, check a sample of old words. If last month's words keep failing, add a review slot or lengthen the list you draw from. A short daily reading routine gives the review a natural place in each lesson.
How iRead handles this
In iRead, four word games are regenerated every 24 hours from the vocabulary of the story the class is reading, and a quiz runs after every session. Every game clear and quiz answer feeds one progress record per word. A word cleared on two different days is marked Mastered, so one good day is not enough. Teachers see retention per word, per game and per class, which shows which words should come back into review.
The core principle: Bring each word back after real gaps, and let some gaps run to weeks. How long the words must last decides how long the gaps should be.
See it in iRead: Four word games are regenerated every 24 hours from the vocabulary of the story the class is reading. A word cleared on two different days is marked Mastered.
See the four gamesKey takeaway
Space vocabulary reviews over days and weeks, make every review a recall, and do not worry about the exact pattern of gaps. A word bank and a five-minute review are enough to start.
Frequently asked questions
How long should the gaps be in spaced repetition?
It depends on how long the words must last. In Cepeda and colleagues (2008), the best gap was about 20 to 40% of a one-week test delay, and 5 to 10% of a one-year delay. For classroom vocabulary, the next lesson, a week later and about a month later is a reasonable start.
Is expanding spacing better than equal spacing?
Not clearly. Nakata (2015) found a small advantage for expanding gaps, Karpicke and Bauernschmidt (2011) found none, and Kim and Webb's (2022) meta-analysis found the two statistically equivalent. The total amount of spacing matters more than the pattern, so choose whichever schedule you can keep.
Can you do spaced repetition without an app?
Yes. Keep the class's key words in a list by week, and open each lesson with a five-minute recall review drawn from the last lesson, last week and about a month ago. Mark the words most pupils miss and bring them back sooner.
Does spaced repetition work for grammar as well as vocabulary?
Probably, though the evidence is less direct. Kim and Webb (2022) found that the size of the spacing effect in second language learning varied with the learning target. Spacing grammar practice over weeks is a sensible default, with varied tasks rather than the same rule drill each time.
Why do pupils prefer cramming?
Because cramming feels better. Massed practice gives quick, fluent recall during the session, which feels like learning. Kornell (2009) found spacing beat massing for 90% of participants, yet 72% believed massing had worked better. Showing pupils their own delayed results can help change their minds.
Sources
- Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354-380. doi.org/10.1037/0033-2909.132.3.354
- Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095-1102. doi.org/10.1111/j.1467-9280.2008.02209.x
- Kim, S. K., & Webb, S. (2022). The effects of spaced practice on second language learning: A meta-analysis. Language Learning, 72(1), 269-319. doi.org/10.1111/lang.12479
- Nakata, T. (2015). Effects of expanding and equal spacing on second language vocabulary learning. Studies in Second Language Acquisition, 37(4), 677-711. doi.org/10.1017/S0272263114000825
- Karpicke, J. D., & Bauernschmidt, A. (2011). Spaced retrieval: Absolute spacing enhances learning regardless of relative spacing. Journal of Experimental Psychology: Learning, Memory, and Cognition, 37(5), 1250-1257. doi.org/10.1037/a0023436
- Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297-1317. doi.org/10.1002/acp.1537
- Dunlosky, J., Rawson, K. A., Marsh, E. J., Nathan, M. J., & Willingham, D. T. (2013). Improving students' learning with effective learning techniques: Promising directions from cognitive and educational psychology. Psychological Science in the Public Interest, 14(1), 4-58. doi.org/10.1177/1529100612453266