Interleaved Practice: Why Mixed Sets Feel Worse but Work

Textbooks teach in blocks. Read section 4.2 on the quadratic formula, then do problems 1 to 20, all of which need the quadratic formula. By problem 5 you are on autopilot, and it feels like mastery. On the exam, though, the problems arrive mixed and unlabeled, and the hardest part is no longer solving them. It is knowing which tool each one needs. Interleaved practice trains exactly that skill by mixing problem types from the start.

In a blocked set, the chapter title tells you the strategy. You only practice executing it. In a mixed set, each problem forces two steps: identify the type, then solve. That first step is what exams test and what blocked homework never practices.

In a 2007 experiment by Doug Rohrer and Kelli Taylor at the University of South Florida, college students learned to find the volume of four kinds of geometric solids. One group practiced in blocks, one kind at a time. The other group got the same problems shuffled. During practice the blocked group looked far better, 89% correct against 60%. One week later, on new problems, the result reversed sharply: 63% for the mixed group, 20% for the blocked group.

This was not a one-off lab result. In a classroom study of 140 seventh graders, students practiced over nine weeks and took a surprise test two weeks later. Material learned with interleaved practice scored 72%, compared with 38% for blocked practice. The effect held even though the problem types looked quite different from each other.

Interleaving also helps you learn categories. In a 2008 study by Nate Kornell and Robert Bjork, people studied paintings by 12 artists, six paintings each, either grouped by artist or mixed. The mixed group was better at naming the artist of new paintings. Most participants still believed grouping had worked better, even after their own test results showed the opposite. In medical training, a study cited in the 2013 Dunlosky review found students who practiced mixed ECG diagnoses scored 47% on new cases, against 30% after blocked practice.

That feeling is the trap. Smooth practice looks like learning, so students and even teachers tend to prefer blocking. A 2022 survey study found that most undergraduates judged schedules with little spacing or mixing to be the most effective, and rated the mixed ones as harder and less enjoyable. Interleaving is a classic desirable difficulty: worse during practice, better when it counts.

• Before you understand the basics. Mixing three methods you have never seen is just confusing. Do a short blocked set first to learn each one.
• Simple fact lists. Studies on mixing vocabulary by category have shown little or no benefit. Spacing matters more there.
• Categories that are very varied inside. When members of one category look very different from each other, some research suggests a little blocking helps you see what they share.
• Unrelated subjects at random. Mixing chemistry with history in one problem set does little. Mix topics that compete for the same kind of question.

Examples by subject

• Math: mix integration by parts, substitution and partial fractions in one set.
• Physics: mix energy, momentum and force problems so you must choose the principle.
• Chemistry: mix limiting reagent, percent yield and molarity questions.
• Biology: mix diagrams of mitosis and meiosis stages and identify each one.
• Languages: mix grammar points, such as past tenses, in one translation drill.

Here is what that looks like for a real week. Say your chemistry unit covers stoichiometry, gas laws and solution concentration. Monday and Tuesday you learn the gas laws with a short blocked set. From Wednesday on, every homework session is twelve problems: four gas law problems, four from last week's stoichiometry and four on concentration, shuffled. You will be slower on Wednesday than you were on Tuesday. By Friday, you will start to notice the clues in each question that point to the right method, such as a pressure and a volume pointing to the gas laws. That noticing is the skill the exam rewards.

Interleaving pairs naturally with spaced repetition: when you mix today's topic with last week's, you are both spacing the old one and practicing discrimination. And every mixed problem is a recall attempt, which is the heart of active recall. For more on building exam-style sets, see how to make a practice test.

If you do not have a ready-made mixed set, you can make one from your materials. In LearnKit, add notes from two or three chapters and generate a quiz; the questions come back from all of them, so you have to work out which idea each one is testing.