Interleaved Practice: did you know that one learns more by alternating subjects of study?

What is interleaved practice?

Interleaved practice consists of alternating the training of different contents in the same subject. For that purpose, the training tasks are organised in a sequence that prevents the consecutive presentation of two exercises of the same type (figure below). This alternation can be used for the consolidation of different contents (such as training different types of arithmetic operations), or for the consolidation of sub-categories of contents (such as training different algorithms for solving an operation).

It has long been known that teaching must be organised, progressive, and systematic. It is not possible to learn to subtract before having spent some time learning to add up. And since what you learn is built on what you have already learned, it is normal and healthy for teaching to be organised by themes. But something different happens with training and recovering topics. There, the results of the research are clear: interleaved practice offers great benefits over blocked practice (as evidenced by the following benefits). ^{1, 2, 3, 4, 7}

What subjects does it apply to?

In the field of mathematics, interleaved practice accumulates evidence of effectiveness at all levels of teaching.^{5, 6, 7, 8} Gradually, research has extended to other fields and has shown the usefulness of this strategy in training motor skills (such as the practice of a musical instrument or a sports skill) and in consolidating the learning of content based on semantic categories, such as the grammar and vocabulary of a foreign language, or categorical content in the sciences and arts (such as the classification of chemical compounds or artistic styles).^{4, 9, 10, 11, 14, 15}

What are the benefits of this strategy?

3-in-1 effect: it improves learning, stimulates reasoning, and promotes attention.

1. Benefits in terms of learning: more knowledge for a longer period of time

When compared to blocked practice, interleaved practice is a more effective strategy for consolidating knowledge and it increases the duration of the learning process. In a study carried out in Mathematics, with 126 seventh-grade students, the researchers compared the effect of interleaved practice and blocked practice in the learning of linear equations and slope calculation. Once the contents were taught, the students carried out a review exercise, which was followed one day later and 30 days later by an incidental assessment task (i.e. carried out without prior notice). The results showed that, for the same learning period, the classes that alternated the subjects' training (interleaved practice) showed significantly greater knowledge and less forgetting than the classes that trained the contents in a sequential and independent way (blocked practice) (picture on the side).⁸

Recently, the extension of this study to 54 seventh-grade classes delivered similar results in the final evaluation of four types of subjects: graphs, inequalities, expressions, and circles.⁶

One of the reasons for the benefits of interleaved practice lies in the process of alternating between the retrieval of current learning experiences (which are active in working memory) and the retrieval of previous learning experiences (stored in long-term memory). This process favours the consolidation of memory traces and reinforces the learning of current and previous contents. Blocked practice, on the other hand, promotes only the intensive learning of recent content. In this sense, the interleaved practice accumulates the benefits of spacing and retrieval strategies. ^{2, 5, 6}

2. Benefits in terms of inductive reasoning

One of the limitations of blocked practice is that it essentially promotes training in the application of a particular response or strategy to solve a task (as happens in massed training of the same type of mathematical exercises or of a particular verbal tense – to the extent that it often makes it unnecessary to read the statement).⁶ Since the solving strategy is previously defined or easily identifiable, this application training favours the mechanisation of the response, but weakens the understanding of the construct that corresponds to it (for example, subtraction or verbal tense). On the other hand, interleaved practice, by diversifying the trained contents, requires students to identify and select a resolution (or response) strategy before proceeding to its application (as is the case, for example, in alternate training of addition and multiplication problems, or various types of grammar exercises).¹³ This analysis mobilises skills such as comparing content, identifying patterns, anticipating results and deciding on the strategy to be applied, favouring not only the application of knowledge but also the inductive reasoning ability. ^{8, 14, 15}

As an example, in a study on the effects of the type of practice on the learning of categories of organic compounds, students who alternatively trained distinct categories performed better in a task consisting of the classification of new compounds (i.e., non-studied categories) than students who blocked each of the studied categories.14 These results replicated, with scientific contents, the results of a previous study which had analysed the learning of painting styles of different painters. In that study, when the participants alternately⁶ studied different styles (interleaved practice), they performed significantly better in identifying the authors of new paintings (never before presented) than the participants who had studied other⁶ styles sequentially (blocked practice).¹⁵ Overall, the research points to the benefits of interleaved practice, not only for the consolidation of the contents, but also for the accomplishment of inductive learning in future tasks.

3. Benefits in terms of attention

In addition to promoting the automation of responses, blocked practice, or repeated training of the same contents, reduces the need for attentional investment in the task. This decrease in attentional demand leads to the demobilisation of cognitive resources (such as memory resources) and hinders the consolidation of learning. In addition, the prolonged performance of similar tasks impairs the ability to focus attention and increases the risk of mental rambling. In the opposite sense, the alternation of content, in interleaved practice, successively updates the novelty of stimuli and the cognitive challenge, contributing to the maintenance of attention and persistence in tasks. ^{8, 11}

How does one apply interleave practice?

Requirements

For effective implementation, research recommends:

1. monitoring the degree of initial learning of the content to interleave, ensuring its acquisition. It is not, of course, a question of practicing topics that do not yet have a minimum of consolidation. Depending on the complexity of the contents and the level of previous knowledge of the students, a more or less intensive and sequential approach may be necessary for the acquisition of the topics, followed by the interleaved practice for their consolidation; 2, 5
2. the timely and informative correction of performance in training tasks to ensure error-free learning and knowledge consolidation.⁶

Some examples of application include:

1. alternating the training of subtopics of the same content. For example, alternating the volume calculation of different geometric solids instead of practicing each formula sequentially and independently,⁵ or, in the case of a foreign language verb, alternating the learning of different forms of conjugation of the past tense¹⁰;
2. alternating the consolidation of different contents. For example, alternating the calculation of the volume of geometric solids with the resolution of fractions.⁵ To achieve that, tasks should be ordered in such a way as to prevent the consecutive presentation of two identical contents;
3. interleaving the training of contents with (self) evaluation and monitoring of learning exercises, enhancing the joint benefits of the interleaved practice and the practice of retrieval;⁶  
4. applying interleaved practice in self-study, either by integrating the strategy into the consolidation exercises provided by the teacher, or by teaching the strategy to the students as a study method; ¹
5. adopting the remote interleaved practice for the contents whose learning is intended to be prolonged, interleaving the learning of new contents with the spaced training of previous contents, combining the benefits of the interleaved practice and the spaced practice; ⁵
6. reorganising the teaching materials for the interleaved consolidation of the contents, taking into account that research shows the predominance of blocked practice in the organisation of teaching materials (such as manuals and workbooks, which are based on the introduction of a specific content, followed by its intensive training, after which new content is introduced). This organisation of the materials conditions the pedagogical and study strategies, and obstructs the benefits that derive from the interleaved practice. ^{5, 6, 8}

Feasibility and ease of application

Finally, recent research evidence points to a positive perception by teachers of the effectiveness and feasibility of this strategy. Even in the absence of prior specific training, the teachers of the aforementioned study by Rohrer et al. (2019) considered the strategy to be easy to apply while teaching Mathematics and also effective and useful for most students, including those with low academic performance.⁶