Interleaving: A Classroom Experiment

Interleaving: A Classroom Experiment

The learning phase took place over 9 weeks in the classroom. During this time, the students received their normal lessons and assignments. There were four different types of problems that were a part of the experiment, and the students’ assignments were constructed so that across the nine weeks they saw 12 different problems of each of the four types. All students learned two types of problems interleaved throughout the assignments, and they learned the other two types of problems blocked in the assignments. However, the problems assigned to these two conditions (interleaving and blocking) were counterbalanced, meaning that across the entire experiment half of the students interleaved each type of problem and half blocked each type of problem. Counterbalancing ensures that any performance differences are not due to some types of problems being easier or harder for the students, and shows us true differences between the two different conditions in the experiment.

When a type of problem was learned through blocking, all 12 practice problems for the students to solve appeared in the same assignment. When a type of problem was learned through interleaving, the first four practice problems for the students to solve appeared in one assignment, and the remaining eight practice problems were distributed across the remaining assignments. In other words, these last eight problems were all mixed up in later assignments to create interleaving.

The researchers rearranged the problems within the assignments to reflect the appropriate blocking and interleaving conditions for students in the math classes, and researchers scored each assignment after the teachers collected them for the purposes of the experiment.

The assessment phase

Two weeks after the none-week learning phase ended, the students were given a surprise assessment test. The test was a surprise so that students would not cram before the test. The researchers created the test, and the teachers did not know what was on it prior to administering it. This procedure ensures that the students weren’t exposed to the problems before the test.

(As an aside, this does not mean researchers don’t trust teachers! We do this in the lab, too. Keeping as much of the procedure “blind” as possible just helps make sure no one accidentally or implicitly biases the results.)


The results are short and sweet! Interleaving practice resulted in test performance that was almost twice as high as blocking practice. Students earned 72% on the problems they interleaved, whereas they earned 38% on the problems they blocked.

This experiment provides another example of effective learning strategies being tested in the classroom, and shows that interleaving even very different types of math problems led to greater learning than blocking them.


(1) Rohrer, D., Dedrick, R. F., & Burgess, K. (2014). The benefit of interleaved mathematics practice is not limited to superficially similar kinds of problems. Psychonomic Bulletin & Review, 21, 1323-1330.

(2) Taylor, K., & Rohrer, D. (2010). The effect of interleaving practice. Applied Cognitive Psychology, 24, 837-848.