Key Ideas
Core concepts
- Mixing different problem types within a session makes students practise selecting the appropriate strategy, not just executing procedures (Rohrer & Taylor, 2007).
- The effort of discrimination and strategy selection is germane cognitive load that strengthens learning (Sweller et al., 2019), and mixed practice improves the ability to apply knowledge in varied contexts (Rohrer, Dedrick, & Stershic, 2015).
Definition
Interleaving Effect: Enhanced learning that occurs when different types of problems or materials are mixed within single study session, as opposed to blocking where one type is practised repeatedly before moving to another (Rohrer & Taylor, 2007).
Connected to
Spacing Effect | Retrieval Practice | Cognitive Load Theory | Problem-Solving | Surface and Deep Structure
Mechanism
Interleaving produces learning benefits through two interconnected mechanisms (Rohrer, 2012). First, students practise not just how to carry out a strategy but selecting the appropriate strategy in the first place (Rohrer, Dedrick, & Burgess, 2014). This discrimination practice is absent in blocked practice, where the problem type is predetermined by the section heading. Second, the mental effort required to determine which method to use strengthens both understanding and memory formation (Sweller et al., 2019). This productive cognitive load builds robust, flexible knowledge rather than rigid procedural memory.
Classroom use
In mathematics, interleaving changes how practice is structured. In statistics, rather than grouping all mean questions together, followed by all median questions, then mode and range, mix these problem types within single study sessions. In trigonometry, combine questions involving sine, cosine, and tangent ratios rather than blocking them. In algebra, blend linear equations, quadratic equations, and inequalities in practice.
Cross-topic integration extends the principle: concepts like fractions, decimals, and algebra appear in multiple mathematical contexts through retrieval practice. Perimeter questions can use fractional dimensions, area problems can require algebraic manipulation, and statistics can work with decimal and fractional data. Students then encounter material in varied contexts rather than isolated topics.
Effective interleaving requires deliberate planning at multiple levels. Within lessons, after teaching a new concept, mix it immediately with previously learned concepts in the practice session, which positions discrimination as central to learning from the start rather than an afterthought. Homework should include the current topic alongside 2-3 previous topics; for example, homework during a statistics unit might include 6 statistics questions, 2 algebra questions, 2 fraction questions, and 2 angle questions. In assessment, mixed-topic quizzes should require strategy discrimination, with section headings removed so students cannot see which method applies to which problem type.
Benefits
Students develop strategy selection skills: they learn when to use different approaches, not just how to execute them, which is the critical skill for independent problem-solving (Rohrer, Dedrick, & Burgess, 2014). This connects to improved discrimination of deep structure beneath surface features, allowing students to distinguish problem types by their underlying principles rather than superficial similarities (Birnbaum et al., 2013).
Mixed practice also creates stronger memory traces than blocked practice, likely because retrieval becomes more effortful and therefore more beneficial (Kornell & Bjork, 2008). Knowledge becomes more flexible and more likely to apply in new contexts where problems do not arrive pre-sorted by type (Pan, 2015). These combined advantages explain why interleaved practice prepares students better for examinations and real-world application (Rohrer & Taylor, 2007).
A desirable difficulty
Interleaving exemplifies desirable difficulties (Bjork, 1994): conditions that slow apparent learning but enhance long-term retention and transfer. The principle creates a metacognitive paradox for students and teachers.
Blocked practice feels easier. When students complete many similar problems consecutively, the method stays active in working memory, so each subsequent problem feels fluent. This creates confidence and apparent mastery, and students leave blocked practice sessions feeling they have learnt effectively. Interleaved practice feels harder. Switching between problem types forces repeated retrieval of different strategies; each problem requires identifying the type and recalling the appropriate method. Students often report frustration and doubt about their learning during interleaved sessions (Bjork & Bjork, 2011).
The ease of blocked practice produces better immediate performance but poorer long-term retention, whilst the difficulty of interleaved practice produces worse immediate performance but better long-term learning (Rohrer, 2012). Teachers and students judge effectiveness by current performance, which leads them to prefer blocked practice despite its inferior long-term outcomes.
The effortful retrieval required by switching between problem types strengthens memory traces more than easy retrieval during blocked practice. Students must actively discriminate between problem types rather than passively following the pattern established by section headings, and this discrimination is the key skill needed for independent problem-solving where problems arrive unsorted (Rohrer, Dedrick, & Burgess, 2014). Varied practice contexts also reduce dependence on specific retrieval cues, making knowledge more flexible and transferable (Birnbaum et al., 2013).
Conditions for success
Timing matters: start interleaving after initial learning is secure (Rohrer, Dedrick, & Stershic, 2015). Students need basic competence with individual procedures before discrimination practice becomes beneficial rather than overwhelming. Premature interleaving can overload working memory before schemas have formed (Sweller et al., 2019).
The metacognitive challenge requires explicit communication. Students need to understand why interleaved practice feels difficult and why that difficulty benefits learning. Without this understanding, they may abandon effective practices because they prefer the fluency of blocked practice (Bjork & Bjork, 2011).
References
Birnbaum, M. S., Kornell, N., Bjork, E. L., & Bjork, R. A. (2013). Why interleaving enhances inductive learning: The roles of discrimination and retrieval. Memory & Cognition, 41(3), 392-402. https://doi.org/10.3758/s13421-012-0272-7
Bjork, R. A. (1994). Memory and metamemory considerations in the training of human beings. In J. Metcalfe & A. Shimamura (Eds.), Metacognition: Knowing about knowing (pp. 185-205). MIT Press.
Bjork, R. A., & Bjork, E. L. (2011). Making things hard on yourself, but in a good way: Creating desirable difficulties to enhance learning. In M. A. Gernsbacher, R. W. Pew, L. M. Hough, & J. R. Pomerantz (Eds.), Psychology and the real world: Essays illustrating fundamental contributions to society (pp. 56-64). Worth Publishers.
Kornell, N., & Bjork, R. A. (2008). Learning concepts and categories: Is spacing the “enemy of induction”? Psychological Science, 19(6), 585-592. https://doi.org/10.1111/j.1467-9280.2008.02127.x
Pan, S. C. (2015). The interleaving effect: Mixing it up boosts learning. Scientific American, 4 August 2015.
Rohrer, D. (2012). Interleaving helps students distinguish among similar concepts. Educational Psychology Review, 24(3), 355-367. https://doi.org/10.1007/s10648-012-9201-3
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(5), 1323-1330. https://doi.org/10.3758/s13423-014-0588-3
Rohrer, D., Dedrick, R. F., & Stershic, S. (2015). Interleaved practice improves mathematics learning. Journal of Educational Psychology, 107(3), 900-908. https://doi.org/10.1037/edu0000001
Rohrer, D., & Taylor, K. (2007). The shuffling of mathematics problems improves learning. Instructional Science, 35(6), 481-498. https://doi.org/10.1007/s11251-007-9015-8
Sweller, J., van Merriënboer, J. J. G., & Paas, F. (2019). Cognitive architecture and instructional design: 20 years later. Educational Psychology Review, 31(2), 261-292. https://doi.org/10.1007/s10648-019-09465-5