Study Skills FundamentalsLesson 3 of 48 min read

Spacing and Interleaving: When to Review

Why spreading reviews out beats cramming, a simple review schedule, and how mixing problem types prepares you for exams.

Lesson 2 covered how to study: test yourself. This lesson covers when and in what order. Two ideas do most of the work here: spacing (spreading reviews out over days) and interleaving (mixing different topics or problem types in one session).

Spacing: the same hours, better results

Imagine two students who each spend six hours on a topic. One does all six hours the night before the exam. The other does one hour on six different days. On the exam the next morning, they might score similarly. A week later, the second student remembers far more.

This is the spacing effect, and it is one of the most studied findings in learning research. A 2006 meta-analysis by Nicholas Cepeda and colleagues, published in Psychological Bulletin, pulled together hundreds of experiments on verbal memory and found that spaced study consistently beat massed study. It also found that the best gap between sessions depends on how long you need to remember: the longer you need to keep something, the longer the gaps can be.

Why does it work? A popular explanation is that each time you return to a topic after a gap, you have partly forgotten it, and the effort of recalling it again strengthens the memory more than reviewing something that is still fresh.

A simple review schedule

You don’t need software or exact intervals. A rough, expanding schedule works for most courses:

SIMPLE REVIEW SCHEDULE (after you first learn a topic)
Review 1   next day             short recall, 10-15 min
Review 2   2-3 days later       recall + fix gaps
Review 3   about 1 week later   mixed with other topics
Review 4   2-3 weeks later      practice questions
Then       before the exam      timed practice

This is a practical rule of thumb, not a precise formula from the research. The principle is what matters: return to each topic several times, with gaps that grow as you get more confident.

Each review should be an active recall session from Lesson 2, not a reread. Our guide to spaced repetition without fancy software shows how to track this on paper, and Leitner boxes do the same job for flashcards automatically.

Interleaving: mix it up

Most textbooks give you twenty problems of the same type after each lesson. That is blocked practice: AAAA BBBB CCCC. You know the method before you read the question, because it’s the one you just learned.

Interleaving mixes the types: A C B A B C. It feels harder and slower, because each problem forces you to decide which method to use. That decision is exactly what an exam asks of you.

A 2007 study by Doug Rohrer and Kelli Taylor, published in Instructional Science, tested this with maths problems. Students who practised with mixed problem types did worse during practice but did much better on a test one week later than students who practised in blocks. The Dunlosky et al. (2013) review rated interleaved practice as “moderate utility”, with the strongest evidence in maths and skill learning.

BLOCKED (feels easy)        INTERLEAVED (feels harder, sticks)
1 Chain rule                1 Chain rule
2 Chain rule                2 Product rule
3 Chain rule                3 Implicit differentiation
4 Product rule              4 Chain rule
5 Product rule              5 Quotient rule
6 Product rule              6 Product rule

How to interleave without getting lost

  • Learn first, then mix. Use blocked practice briefly when a method is brand new. Mix once each method roughly works.
  • Mix related things. Interleave problem types within a subject (different integration methods), not completely unrelated subjects in one ten-minute block.
  • Shuffle your flashcards instead of studying them in chapter order.
  • Label each problem before solving it. Write “which method?” next to the question, then solve. This trains the choosing skill directly.

Our guide on how to study for a math test shows a full mixed practice set.

Putting both together

A good week combines both ideas. Each session reviews older topics (spacing) and mixes problem types (interleaving):

SESSION PLAN (50 min)
10 min   Recall: last week's topic (spaced review)
25 min   Mixed practice: 3 problem types, shuffled
10 min   Check answers, update error log
5 min    Note which topics need a review in 2-3 days

Key takeaways

  • Spreading the same study time over several days beats cramming for anything you need to remember beyond tomorrow.
  • Use an expanding schedule: next day, a few days later, a week later, then weeks later.
  • Interleaving (mixing problem types) feels harder but builds the skill of choosing the right method.
  • Combine both: every session reviews something older and mixes types.

Sources checked: 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). Rohrer, D., & Taylor, K. (2007), The shuffling of mathematics problems improves learning, Instructional Science, 35. Dunlosky et al. (2013), Psychological Science in the Public Interest, 14(1).

Lesson 4 puts everything into a schedule you can follow.

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