Make Learning Stick

Brad Kelly explains why spaced practice and interleaving benefit long term learning and performance gains and could supercharge the learning in your classroom.

After getting an absolute drubbing one Saturday morning, my son’s annoyed rugby league coach warned his players that the following Tuesday night’s footy practice would be devoted solely to tackling practice. The boys would spend a mid-winter’s night hitting tackling bags in an effort to prevent a repeat performance that let in nearly six tries.

But I began to wonder if focusing on tackling would be the silver bullet the coach had hoped. After all, footy is also about kicking, passing and catching, team coordination, and I suspected that the coach’s heavy focus on tackling drills would fail to yield the result he was looking for.

Why? Spaced practice and interleaving.

Have you ever struggled with a new skill or problem and after repeated attempts, walked away in frustration only to come back a few days later and seem to be able to ‘get it’ with ease?

It’s got to do with spaced practice and interleaving. These two cognitive processes have been shown in the research to boost performance, increase memory storage and deepen understanding, and with a bit of planning, they could be two of the most powerful tools in your teacher toolkit.

Spaced practice – consistency over intensity

 While many students cram (or mass learning to get technical) before an exam, the research has consistently supported the idea that spacing study over a long period of time is more effective for making learning stick.

Ever since the German researcher Hermann Ebbinghaus set himself the task of remembering a series of nonsensical syllables and testing how many times it took him to recite the sequence correctly and tracked the distance between each study session to measure the most effective study method, cognitive psychologists have studied the phenomenon in classrooms all around the world.

Researchers have found that spacing out learning in days or weeks rather than a single study session had significant gains for memory retrieval and understanding. And this insight holds for just about any knowledge acquisition or skill development from learning a language to playing the piano.


At its heart, Carpenter and Agarwal (2020) suggest that “when information is quickly acquired, it’s quickly forgotten.” That’s obvious, because it is not the short-term memory’s role to retain information.

But by inserting a delay between each study session, and asking students to retrieve earlier information, it is believed that this increases ‘storage strength’ rather than ‘retrieval strength.’ Think of it like going to the gym to exercise our memory muscle (more on retrieval practice in the next article).

Spacing is counterintuitive for two reasons. First, students believe that it makes more sense to study intensely prior to a test because the distance between the study and the test is shorter. While there are short term gains from that, students will usually lose the information soon after.

The second, is that spacing is a “desirable difficulty.” It just feels harder to retrieve information from the memory in spaced periods.  However, retrieval (rather than re-reading notes) actually benefits learning for memory, understanding and skills and knowledge transfer.

The length between study sessions could be minutes, days, weeks or months. It depends on the professional judgement of the teacher about the appropriate delay for the content.

To implement spaced practice in the classroom, teachers could create a ‘deck of cards’ with revision material on them for retrieval practice; stagger homework activities across all topics throughout the year so that students do not ‘compartmentalise learning’; or begin each class with a short quiz that includes a question from the last lesson, the last week, the last month, and the last term.

Interleaving – same, same but different

Interleaving involves studying two or three related topics in a single study session (this is important to distinguish it from spacing) rather than spending too long on a single topic, known as blocked practice. This encourages students to better discriminate between related ideas.

For example, in a study of 1920s and 1930s Germany, students could examine the detail of the Dawes Plan and the Young Plan in the same session in order to strengthen their understanding by identifying the similarities, differences and contexts.

Further, researchers show that varying the order in which ideas are studied strengthens student understanding by allowing students to build more connections and categorisation. What makes interleaving beneficial is the similarity of material. 

While interleaved practice leads to more inaccuracies in the short term than blocked practice, it has demonstrated greater accuracy and performance over time.

Weinstein et. al (2018) report a study in which a group of 4th graders were given various formulas to calculate the features of three-dimensional objects. In the blocked conditions, student performance fell from 100 percent to 38 percent the next day.

In the interleaved conditions where students practised various formulas, performance went from 81% to 78% in the space of a day. That is, while interleaving, like spaced practice is a “desirable difficulty”, it is associated with short term failure but long-term benefits.

In the words of Dylan Wiliam, sometimes before you get better, you have to get a little bit worse.


The research jury is still out, but some believe that studying related material strengthens the student’s ability to remember concepts through examples and counterexamples. The second core belief is that interleaving allows students to select correct strategies for solving a problem because it mirrors real life (we are never forced to answer multiple questions on the same thing in a row) and the use of incorrect strategies allows them to pick a better strategy.

The problem with interleaving is that we do not yet know the degree to which the relatedness of material is effective and it is also hard to differentiate it from the gains of spacing.

Whatever the case, with both spaced practice and interleaving, with a little bit of planning on behalf of the teacher (it’s unlikely students will plan this out in fine detail), these two cognitive processes are likely to supercharge learning gains.