Spaced repetition in language learning: what does the research say?

Spaced repetition means repeating the same word or structure with time in between rather than back to back in one session. It is one of the most robust findings in cognitive psychology, and it holds for second language learning too: a 2022 meta-analysis combining 48 experiments found a medium-to-large effect for spaced practice. The best gap is not fixed, though; it depends on how long you want to remember what you learned.

Spacing and the length of the gap: two separate questions

Researchers separate two things here. The spacing effect says that spreading repetitions over time beats doing them back to back; this finding is very consistent. The lag effect claims that longer gaps beat shorter ones; this finding is much less consistent. So “spread out your review” is solid advice, while “the longer the gap, the better” is not.

What did the research find?

Kim and Webb's 2022 meta-analysis combined spaced-practice experiments in second language learning: 48 experiments, 98 effect sizes and 3,411 participants. The findings fit into four points: spaced practice improved learning with a medium-to-large effect; shorter gaps were as good as longer ones on an immediate test but fell behind on a delayed test; equal gaps and gradually expanding gaps did not differ significantly; and the results varied with what was learned, the number of sessions and the type of activity.

Raquel Serrano's review, published the same year, examined 47 studies with a focus on the classroom. It notes that the spacing effect is consistent for vocabulary, but that classroom studies have generally not confirmed an advantage for longer gaps. There are signs that longer gaps help only when knowledge is tested after a long delay, which fits general memory research: according to the large 2006 review by Cepeda and colleagues, the best gap grows as the time you need to remember grows.

Is it the same for grammar, pronunciation and fluency?

Not exactly. According to Serrano's review, in grammar, mixing different structures in the same lesson (for example the past simple next to the present perfect) gives longer-lasting results than spending the whole lesson on one structure. On gap length for grammar the evidence conflicts: longer gaps may suit recognition knowledge, shorter gaps may suit making a structure automatic in speech. For speaking fluency and pronunciation rules, some studies point to more concentrated practice with short gaps being more useful.

What it means for learners

This schedule is an example, not the “best” schedule found by research; the research does not point to one right gap. What matters is two principles: spread your review over several days and weeks, and time your last review to how far ahead your exam or trip is. For a practical way to keep your words together, see how to keep an English vocabulary notebook.

What it means for teachers

Bringing back the words taught in class through short checks in later lessons works better than one long review; Serrano suggests cumulative quizzes that include earlier lessons' words for this reason. Comparing two similar structures in the same lesson and giving a structure more than one lesson are also among the review's teaching suggestions. For speaking fluency, repeating the same speaking task at short intervals is worth considering.

The limits of the research

According to Serrano, some of the meta-analysis's sub-analyses rest on few studies and participants, so some results are inconclusive. Much of the general memory research behind the spacing effect was also done with university students and verbal material. Because “short” and “long” gaps are defined differently from study to study, comparing results is difficult. The learner's level and individual differences also affect the outcome. So spaced repetition is a strong principle, not one schedule that fits everyone.

For a practical summary of the topic without the research detail, see why new words don't stick.

Sources

Kim, S. K., and Webb, S. (2022). The effects of spaced practice on second language learning: A meta-analysis. Language Learning, 72(1), 269–319. https://doi.org/10.1111/lang.12479 · Serrano, R. (2022). A state-of-the-art review of distribution-of-practice effects on L2 learning. Studies in Second Language Learning and Teaching, 12(3), 355–379. https://doi.org/10.14746/ssllt.2022.12.3.2 · Cepeda, N. J., et al. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin.

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