You spend three hours practising quadratic equations. You get faster and more accurate with each problem. The next night you spend three hours on systems of linear equations. Same result — by the end of the session, you are nailing them.
Then the exam arrives with a mix of both problem types, and you freeze. The problems look similar, but you cannot figure out which approach to use. Your hours of practice feel wasted.
This is the hidden flaw of blocked practice — studying one topic at a time until you feel confident, then moving to the next. It feels effective during practice but falls apart on mixed tests. There is a better approach: interleaving, the technique of mixing different topics within a single study session. It feels harder in the moment, but the research shows it can double or even triple your test performance.
What Is Interleaving?
Interleaving is the practice of alternating between different topics, problem types, or skills during a single study session instead of focusing on one at a time.
The contrast is simple:
- Blocked practice: AAABBBCCC — study all of topic A, then all of topic B, then all of topic C
- Interleaved practice: ABCABCABC — mix topics A, B, and C throughout the session
For example, instead of spending an hour on multiplication, then an hour on division, then an hour on fractions, you would mix multiplication, division, and fraction problems throughout the entire study session.
This applies to any subject. A language learner might interleave vocabulary, grammar, and reading comprehension rather than doing each in isolation. A medical student might mix questions about cardiology, pulmonology, and nephrology instead of studying one organ system at a time.
The critical detail: interleaving means mixing related topics that you could confuse with each other. The goal is to practise distinguishing between similar concepts — not to randomly jump between unrelated subjects.
The Science Behind Interleaving
Interleaving is one of the most counterintuitive findings in learning science. It feels worse during practice, yet it produces significantly better results on tests.
Kornell and Bjork (2008) — The Painting Study
In a landmark experiment, Kornell and Bjork had participants learn the painting styles of 12 different artists. One group studied paintings in blocks — six paintings by artist A, then six by artist B, and so on. The other group saw the paintings interleaved — one by artist A, one by artist B, one by artist C, then back to A.
On a later test where participants had to identify the artist of paintings they had never seen before, the interleaved group scored 59 percent compared to just 36 percent for the blocked group — a 64 percent improvement.
Here is the remarkable part: despite performing significantly better, most participants in the interleaved group believed blocked practice had been more effective. The technique that felt harder actually produced dramatically superior learning.
Rohrer and Taylor (2007) — Mathematics
Rohrer and Taylor tested interleaving with math students learning to calculate the volumes of different solid shapes. Students either practised one shape type at a time (blocked) or mixed problem types (interleaved).
On a test one week later, the interleaved group scored three times higher than the blocked group. When every practice problem is the same type, students can coast on autopilot without thinking about which formula to use. Interleaving forces them to identify the problem type before they can solve it — which is exactly what a real test requires.
The Contextual Interference Effect
The benefits of interleaving are explained by a broader phenomenon called contextual interference. When you switch between tasks during practice, each switch creates a small amount of interference that forces your brain to reload the relevant strategy or concept.
This reloading process is effortful, which is why interleaved practice feels harder. But that effort strengthens your ability to discriminate between similar concepts and select the right approach — skills that blocked practice never exercises.
The contextual interference effect was first identified in motor learning research by Shea and Morgan (1979), who found that participants who practised three different arm movements in mixed order outperformed those who practised each movement in blocks. The principle generalises across domains: for any task where you need to choose between multiple approaches, mixing during practice beats isolating during practice.
Why Interleaving Works: Four Mechanisms
Interleaving does not just add variety to your study sessions. It engages specific cognitive processes that blocked practice skips entirely.
1. Discriminative Contrast
When you study topic A alongside topics B and C, you are constantly comparing and contrasting them. This helps you notice the features that make each topic unique — the differences that tell you "this is a quadratic, not a linear equation" or "this painting is an Impressionist, not a Baroque."
Blocked practice never exercises this skill because every problem is the same type. You never have to ask which approach to use — you already know.
2. Retrieval Practice
Each time you switch topics during interleaved study, you must retrieve the relevant knowledge from memory. When you move from a chemistry problem to a biology problem and back, you are practising retrieving chemistry knowledge multiple times across the session — each time after a brief gap filled with different material.
This is a natural form of retrieval practice, which research consistently shows is one of the most powerful learning strategies available.
3. Strategy Selection
Real exams and real-world problems do not come with labels. You have to figure out what type of problem you are facing before you can solve it. Interleaving trains this crucial meta-skill by forcing you to identify the problem type at each step.
Blocked practice lets you skip strategy selection entirely because the problem type is predetermined. This creates a false sense of competence — you can solve problems when you already know the category, but not when you have to figure it out yourself.
4. Desirable Difficulty
Psychologist Robert Bjork's concept of desirable difficulty explains why harder practice leads to better learning. Interleaving adds difficulty to each practice attempt — you must work harder to recall the right approach and execute it — and this increased effort deepens encoding.
Easy practice produces fast performance gains that evaporate quickly. Difficult practice produces slower gains that last. Interleaving sits squarely in the desirable difficulty sweet spot: hard enough to strengthen learning, but not so hard that you cannot make progress.
Common Misconceptions About Interleaving
"It's just multitasking"
Multitasking means doing two things simultaneously — texting while studying, for instance. Interleaving means doing one thing at a time but switching which thing you are doing throughout the session. You give full attention to each problem; you simply vary the problem type. There is no divided attention involved.
"If it feels hard, I must be doing it wrong"
This is the biggest barrier to adopting interleaving. Blocked practice feels smooth and productive because you build momentum within a single topic. Interleaving feels choppy and frustrating because each switch requires mental effort. But that friction is the learning. Kornell and Bjork's study showed that learners consistently rated interleaving as less effective even when their test scores told the opposite story.
"I should mix completely unrelated subjects"
Interleaving works best when you mix related topics that share surface features but differ in their underlying structure or approach. Mixing quadratic equations with systems of equations makes sense — they look similar but require different methods. Mixing quadratic equations with French vocabulary does not, because there is nothing to discriminate between.
"Blocked practice is always worse"
Blocked practice has its place — specifically, during your very first encounter with brand-new material. When you have zero familiarity with a concept, you need focused attention to build a basic schema. Once that initial framework exists — even if it is fragile — interleaving becomes the superior strategy for strengthening and refining it.
"Interleaving only helps with memorisation"
Interleaving actually helps more with higher-order skills like problem-solving and transfer than with simple memorisation. Its primary benefit is teaching you to select the right approach and apply knowledge flexibly — skills that go well beyond rote recall.
How to Use Interleaving in Your Studies
Math and Quantitative Subjects
Instead of practising 20 problems of the same type, mix problem types within a single practice set. If you are studying for a calculus exam, interleave integration techniques: substitution, integration by parts, partial fractions, and trigonometric substitution. The key challenge on calculus exams is usually identifying which technique to use, not executing it — and interleaving trains exactly that skill.
If you are preparing for the SAT Math section, shuffle problems from different categories in each practice session. For tips on structuring cards that work well in mixed review, see our guide on creating effective flashcards.
Languages
Alternate between vocabulary, grammar, reading, and listening within each study session. If you are learning verb tenses, practise present, past, and future tense conjugations in mixed order rather than mastering one before moving to the next.
Sciences
Mix problems from different chapters or topic areas. For AP Chemistry, interleave stoichiometry, equilibrium, and thermodynamics problems. For Organic Chemistry, mix reaction types rather than studying each mechanism in isolation.
Test Preparation
Create practice tests that mirror the mixed format of real exams. Most standardised tests do not group questions by topic — they deliberately mix subjects to test your ability to switch between contexts. Your practice should do the same.
Interleaving + Spaced Repetition + Active Recall
Interleaving becomes even more powerful when combined with two other evidence-based techniques: spaced repetition and active recall.
Here is how the three techniques work together:
- Active recall ensures you are retrieving information from memory, not just re-reading it. Each practice attempt is a genuine test of your knowledge.
- Interleaving ensures you are mixing topics within each session, which trains discriminative contrast and strategy selection.
- Spaced repetition ensures your sessions are distributed over time at optimal intervals, which maximises long-term retention.
Think of it this way: active recall determines how you study (by testing yourself). Interleaving determines what you study in each session (a mix of topics). Spaced repetition determines when you study (at gradually increasing intervals).
This three-technique combination is what Sticky is built around. When you review flashcards in Sticky, the app automatically interleaves cards from different subjects, schedules reviews using the spacing effect, and forces active recall on every card. You get all three techniques working together without having to manage any of them yourself.
The research is clear: mixing your study topics feels less comfortable than blocked practice — and that discomfort is the cost of real learning. Start interleaving your practice sessions today, and your next mixed exam will not catch you off guard.
