When practicing a set of related skills or problem types, the instinctive approach is to focus on one at a time: master skill A completely, then move on to skill B, then C. This is called blocked practice, and it feels efficient and orderly. Research suggests that a different, messier-feeling approach often works better. It is called interleaving.

Two Ways to Practice

Suppose you need to practice three types of problem—call them A, B, and C.

Blocked practice tackles them in clean blocks: all the A problems, then all the B problems, then all the C problems. AAA BBB CCC.

Interleaved practice mixes them up: A, then C, then B, then A, then B, and so on. The same problems are practiced, but their order is shuffled rather than grouped.

The interleaving research finds, consistently and somewhat surprisingly, that the mixed-up approach leads to better long-term learning, even though blocked practice often feels more productive in the moment.

Why Blocked Practice Feels Better but Works Worse

Blocked practice feels good because, within a block, performance is smooth. Once you are working through the A problems, you are "warmed up" on A. Each problem feels easier than the last. This smoothness produces a satisfying sense of mastery.

But that sense can be misleading. Within a block, you do not have to figure out which kind of problem you are facing—you already know it is an A. You can apply the A method on autopilot.

This is the hidden weakness. In real life—on a test, or in any genuine application—problems do not arrive neatly labeled and grouped. You have to recognize what kind of problem you are facing and choose the right approach.

Why Interleaving Works

Interleaving builds exactly the skill that blocked practice skips.

When practice is mixed, each new problem forces you to first identify what kind of problem it is and then retrieve the appropriate approach. You cannot run on autopilot, because the problem type keeps changing.

This is harder, and it makes practice feel less smooth. But that difficulty is productive. Interleaving trains not just the execution of each skill, but the crucial ability to discriminate between problem types and select the right strategy. It practices learning in the form it will actually be needed.

A Form of Desirable Difficulty

Interleaving is one example of a broader principle: desirable difficulties. These are conditions that make practice feel harder and slower, while making the resulting learning more durable and more transferable.

Interleaving, like the testing effect and spaced practice, works by adding the right kind of difficulty. The discomfort is not a sign of poor practice; it is a sign that the practice is engaging the mind more deeply.

Practicing the Messy Way

The lesson of interleaving is to distrust the smooth, satisfying feeling of blocked practice and to embrace a messier approach:

Mix related problem types or skills within a practice session.
Expect it to feel harder, and recognize that difficulty as productive.
Practice discrimination, not just execution—learning to tell which approach a situation calls for.

Interleaving is a counterintuitive but well-supported insight from the psychology of learning—a reminder that the most comfortable way to practice is not always the way that builds the most lasting, usable skill.