Most textbooks teach one topic and then give a set of practice problems on that topic: twenty questions on the area of a triangle, then twenty on circles. This is called blocked practice. It feels efficient, because once you know which method to use, every problem after the first is easy.
That is exactly the problem. On a test, problems do not arrive labeled with the method to use. Choosing the right approach is part of the skill, and blocked practice never exercises it.
What interleaving is
Interleaving means mixing different types of problems or topics within one practice session: a triangle, then a circle, then a cylinder, then a triangle again. Each problem forces you to ask "what kind of problem is this?" before you solve it.
The evidence
In a study by Doug Rohrer and Kelli Taylor (2007), college students learned how to find the volume of four kinds of geometric solids. One group practiced in blocks; the other practiced with the problems mixed. During practice, the blocked group did better. On a test a week later, the mixed group scored roughly three times as high.
The effect is not limited to math. Nate Kornell and Robert Bjork (2008) had people learn the painting styles of different artists. Seeing the artists' paintings mixed together led to better recognition of new paintings than seeing each artist's work in a block, even though most participants believed that blocking had helped them more.
That last detail is important. Interleaving feels worse while you are doing it, so students often judge it as less effective. That is one reason it is underused.
An example from a math class
Imagine a unit on area and volume. The usual approach would give students a page of rectangle problems on Monday, circles on Tuesday and cylinders on Wednesday. An interleaved version might look like this:
- Monday: teach rectangles, practice them with a short blocked set.
- Tuesday: teach circles, then practice a mix of circles and rectangles.
- Wednesday: teach cylinders, then practice a mix of all three, in random order.
- Friday: a quiz where every problem type appears, shuffled, plus two problems from last month's unit on fractions.
Students will make more mistakes on Tuesday and Wednesday than they would with blocked sets. Those mistakes are mostly about choosing the wrong formula, which is exactly the error you want them to make, and fix, before the test.
What about other subjects?
In language classes, mix vocabulary and grammar from several units rather than drilling one list. In science, alternate between problem types that use different equations. In history, practice comparing causes across several events instead of studying each event alone. The principle is the same: make students decide what they are looking at before they answer.
When to use it, and when not to
- Block first, then mix. When a method is completely new, a few blocked examples help students understand it. Once they can do it, switch to mixed practice.
- Mix things that are easy to confuse. Interleaving helps most with similar formulas, related grammar rules or painters with similar styles.
- Combine it with spacing. Mixing in problems from earlier units also spreads practice out over time.
How to build interleaved practice
For teachers
- Make homework half on today's topic and half on earlier ones.
- Shuffle end-of-unit review sets instead of grouping them by section.
- In quizzes, mix questions that require choosing a method.
For students
- When doing practice problems, pick them from different sections at random.
- Study two or three subjects in a session and switch every 30 to 45 minutes.
- Write problems on cards without the chapter number, and shuffle them.
A mixed quiz is easy to make if you generate questions from several chapters at once. In Quizbot you can upload up to four files together and get one quiz that draws on all of them. Use the AI test generator for a cumulative practice test that mixes topics, just like the real exam.
The takeaway
- Blocked practice teaches how to use a method. Interleaved practice teaches when to use it.
- Expect mixed practice to feel harder and to look worse during practice. That is normal.
- Judge by the test a week later, not by how the session felt.
