Interleaving and Spaced Repetition: How to Combine Both for Maximum Retention

Combine interleaving and spaced repetition by applying each at a different scale: spaced repetition decides when you review a topic (the timing), and interleaving decides how you order topics within a session (the structure). A shuffled spaced-repetition flashcard deck does both at once — each card returns on its own schedule, and the shuffle mixes topics into interleaved order.

Two techniques consistently appear at the top of every evidence-based study method ranking: spaced repetition and interleaving. Both outperform conventional study methods by significant margins in randomised experiments. Both exploit effortful retrieval as their core mechanism. And importantly, they are not alternatives — they target different aspects of the same learning problem and are most effective when used together.

What each technique does

Spaced repetition solves the temporal problem: without deliberate review at increasing intervals, material is forgotten rapidly and systematically. The forgetting curve (Ebbinghaus, 1885) shows 60–75% of material forgotten within a week without review. Spaced repetition combats this by scheduling reviews at the moment just before forgetting reaches critical levels — each review resets the curve at a higher baseline.

Interleaving solves the contextual problem: blocked practice on a single topic trains execution of a method but not identification of which method to use. Exams present problems from multiple topics in unpredictable order — which is exactly what interleaved practice mimics. The identification step forced by interleaving is the training that transfers to exam conditions.

Together, they address both dimensions: when to review and how to structure review.

The two problems they solve together

Consider a student studying three A Level science subjects: Biology, Chemistry, and Physics.

Without spaced repetition: Material from week 1 (cell biology) is not reviewed until week 6 (just before the exam). By then, most of it has been forgotten and must be re-learned rather than reviewed.

Without interleaving: When the student does review, they revise all biology in one block, all chemistry in one block, all physics in one block. They can execute each subject's methods but struggle to switch between them in a mixed exam paper.

With both: Topics from week 1 appear in review sessions in week 2 and week 4 (spaced). When reviewing in week 2, the student alternates between biology topics from week 1, chemistry topics from week 1, and physics topics from week 1 within the same session (interleaved). The review is both timely (spaced) and contextually discriminative (interleaved).

How to implement them together

Method 1: Spaced schedule with interleaved sessions

Step 1: Build a spaced review schedule. When you first study a topic, schedule a review for the next day, another review one week later, and a third review three weeks later. Use a tracker (spreadsheet, revision diary, or the Study Planner tool) to record these review dates.

Step 2: When review day arrives, interleave. On a given day, you might have three topics due for review (from different subjects or different areas of the same subject). Instead of completing all biology review then all chemistry review, interleave: 20 minutes biology topic A, 20 minutes chemistry topic B, 20 minutes physics topic C, repeat.

The spaced schedule determines which topics are reviewed today. The session structure determines that they are reviewed in interleaved order.

Method 2: Shuffled spaced repetition flashcards

A spaced repetition flashcard deck that is reviewed in shuffled order (which standard SR software does by default) automatically combines both techniques:

• The SR algorithm schedules each card to appear at the optimal spacing interval (spaced repetition)
• The shuffled order means consecutive cards come from different topics (interleaving)

Each flashcard review session is simultaneously a spaced review (cards appear at their due interval) and an interleaved review (consecutive cards are from different topics and require re-identification of context).

Use the Flashcard tool with shuffled mode enabled for the most efficient combined implementation.

Method 3: The interleaved review session with built-in spacing

Structure each study session as three 25-minute blocks on three different topics (interleaved), all of which are topics due for their scheduled review (spaced):

Session (Day 14 of revision):
Block 1: Biology – Cell respiration (due for review 1 from Day 7)
Block 2: Maths – Differentiation (due for review 1 from Day 7)  
Block 3: Chemistry – Organic reactions (due for review 1 from Day 7)

Each block is a retrieval practice session (close notes, write from memory, check against notes). The three topics are due for review at approximately the same time (both were first studied about one week ago), and they are reviewed in interleaved order within the session.

The compounding benefit

The research on each technique in isolation shows large effects:

• Spaced repetition: 1.5–3× better retention at 1 month vs. massed practice (Cepeda et al., 2006)
• Interleaving: 40–78% better performance at 1–4 week delays (Kornell & Bjork, 2008; Rohrer & Taylor, 2007)

When combined, both address different vulnerabilities in the learning process — temporal forgetting and contextual confusion — and their effects are largely additive. A study system using both consistently will outperform a system using either alone.

Practical starting point

The simplest combined implementation for most students:

1. Use the Flashcard tool with shuffle enabled for any subject involving facts, definitions, or procedures
2. When revising from notes, alternate between topics within each session — never the same topic twice in a row
3. Schedule review sessions in a revision calendar so that each topic is touched at approximately 1 day, 1 week, and 3 weeks after initial study

For the deep dive on each technique separately, see Interleaving Study Technique and What Is Spaced Repetition?. For the full interleaving course with practical exercises, see Interleaving.

References

• Cepeda, N.J., et al. (2006). Distributed practice in verbal recall tasks. Psychological Bulletin, 132(3), 354–380.
• Kornell, N., & Bjork, R.A. (2008). Learning concepts and categories. Psychological Science, 19(6), 585–592.
• Rohrer, D., & Taylor, K. (2007). The shuffling of mathematics problems improves learning. Instructional Science, 35, 481–498.
• Bjork, R.A. (1994). Memory and metamemory considerations in the training of human beings. In Metacognition, 185–205. MIT Press.
• Dunlosky, J., et al. (2013). Improving students' learning with effective study techniques. Psychological Science in the Public Interest, 14(1), 4–58.