Most people study one topic at a time until they feel solid on it, then move to the next. It feels like a sensible approach. The data says otherwise.
What is it?
Blocked practice means drilling one skill or topic repeatedly before moving on: AAABBBCCC. Interleaving mixes them: ABCBCABC. Blocked practice feels better during the session because your accuracy is higher and the material comes faster. Interleaved practice feels harder and messier. On delayed tests, the interleaved group consistently wins by a lot.
In a study by Taylor and Rohrer, blocked practice produced 89% accuracy during sessions while interleaved practice produced only 60%. On a test one week later, the interleaved group scored 63% compared to 20% for the blocked group. The advantage completely reversed. A large randomized controlled trial by Rohrer and colleagues involving 787 students across 54 classes found similar results: interleaved practice produced test scores of 61% compared to 38% for blocked practice.
The Neuroscience
Two mechanisms explain why interleaving works. When problems are blocked, the context already signals which method to use, so you only practice executing the procedure. When they're interleaved, you first have to identify what kind of problem you're facing and select the right approach. That selection step is exactly what real tests require, and interleaving gives you practice at it during every session.
The second mechanism is that interleaving forces spacing and retrieval practice on each topic for free. Every time you return to a topic after working on something else, you're retrieving it from a slightly degraded state, which strengthens the memory trace. You get the spacing effect and the testing effect built into every session without any extra effort.
Neuroimaging studies associate interleaved practice with greater frontal and parietal activity, regions involved in strategy selection and working memory. The brain is doing more work, and that additional work is what produces the durable learning.
One caveat worth knowing: for complete beginners, heavy interleaving before a skill is at all established can be overwhelming. A few blocked sessions to get a basic foothold, then interleaving to lock it in, works better than jumping straight into mixing.
Try it this week
Pick a subject where you have multiple topic types or problem types to practice, whether that's math, a language, a musical repertoire, or anything else with variety. Mix the problem types instead of finishing all of one kind before moving on. Expect it to feel harder and slower. That's the mechanism working, not a sign that it isn't.
- Identify one area where you currently practice blocked and experiment with mixing the types this week.
- Expect interleaving to feel slower and harder during the session.
- If you're a complete beginner at something, do a few blocked sessions first, then start mixing.
- Use interleaving especially for review and test prep since that identification and selection practice mirrors what real tests require.
- Compare how you do on the next real assessment versus your practice accuracy. The gap usually tells the story.
