Why the practice that feels worse often works better

Imagine two students practicing math. One does ten problems of one type, then ten of the next. The other gets the same twenty problems shuffled together. The first student feels fluent and the second feels lost. Weeks later, which one remembers more?

Desirable difficulties

Robert and Elizabeth Bjork coined the phrase desirable difficulties for learning conditions that slow apparent progress during practice but improve long-term retention and transfer. Spacing, testing yourself, and mixing topics are the usual examples. The common thread is that they make practice feel harder in the moment, which makes it easy to conclude they are not working.

The Bjorks are careful about the word desirable. A difficulty is only useful if the learner has the background to meet it. Make a task too hard and it stops being a productive struggle and becomes plain failure.

The shuffling study

In 2007, Doug Rohrer and Kelli Taylor compared blocked practice with mixed practice in mathematics and published the results in Instructional Science under the title The shuffling of mathematics problems improves learning. In blocked practice, similar problems are grouped together. In mixed practice, problem types are interleaved. Mixed practice improved learning, particularly on a later test.

One plausible reason is that interleaving forces learners to decide which method a problem calls for, not only how to apply a method they already know is coming. That selection skill is exactly what an exam demands and what a block of identical problems never trains.

An honest note on the evidence

The Dunlosky review rated interleaved practice as moderate utility, not high. The authors found it promising but concluded that the evidence was still limited. That is a fair reading, and it is why we describe interleaving as a strong bet to try, not a guarantee.

How to use it

• When you meet a new concept, practice it in a block until you can do it. You need a foundation before mixing helps.
• Then switch to mixed sets. In a question bank, that means random or mixed mode instead of working one topic at a time.
• Do not judge mixed practice by how it feels. Judge it by how you perform on questions a few days later.
• If mixed sets are collapsing your accuracy, step back to blocks for that topic and return to mixing once it is steadier.