## Key Ideas > [!abstract] Core Concepts > > - **Mix different problem types within sessions**: Practice selecting appropriate strategies, not just executing procedures (Rohrer & Taylor, 2007) > - **Increases germane cognitive load**: Mental effort invested in discrimination and strategy selection strengthens learning (Sweller et al., 2019) > - **Improves knowledge transfer**: Mixed practice improves 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]] --- ## How interleaving works Interleaving produces learning benefits through two interconnected mechanisms (Rohrer, 2012): **Strategy selection practice**: Students practise not just how to carry out a strategy, but selecting the appropriate strategy first (Rohrer, Dedrick, & Burgess, 2014). This discrimination practice is absent in blocked practice where the problem type is predetermined by the section heading. **Increases [[Cognitive Load|germane load]]**: 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. ## Practical applications 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, practitioners should mix these problem types within single study sessions. In trigonometry, questions involving sine, cosine, and tangent ratios should be combined rather than blocked. In algebra, linear equations, quadratic equations, and inequalities should blend together in practice. Cross-topic integration extends this principle by ensuring that concepts like fractions, decimals, and algebra appear in multiple mathematical contexts through [[Retrieval Practice|retrieval practice]]. Perimeter questions can use fractional dimensions, area problems can require algebraic manipulation, and statistics can work with decimal and fractional data. This ensures students encounter material in varied contexts rather than isolated topics. Effective interleaving requires deliberate planning at multiple levels. Within lessons, after teaching a new concept, teachers should immediately mix it with previously learned concepts in the practice session. This positions discrimination as central to learning from the start rather than treating it as an afterthought. Homework design should include the current topic alongside 2–3 different 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 Interleaving produces several advantages over blocked practice. 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 [[Surface and Deep Structure|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). ## Interleaving as 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. Each subsequent problem feels fluent because the strategy remains accessible. This creates confidence and apparent mastery. 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. This creates difficulty and uncertainty. Students often report frustration and doubt about their learning during interleaved sessions (Bjork & Bjork, 2011). **Performance versus learning**: The ease of blocked practice produces better immediate performance but poorer long-term retention. The difficulty of interleaved practice produces worse immediate performance but better long-term learning (Rohrer, 2012). Teachers and students judge effectiveness by current performance, leading them to prefer blocked practice despite its inferior long-term outcomes. **Why interleaving works**: 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. This discrimination practice is the key skill needed for independent problem-solving where problems arrive unsorted (Rohrer, Dedrick, & Burgess, 2014). Varied practice contexts reduce dependence on specific retrieval cues, making knowledge more flexible and transferable (Birnbaum et al., 2013). ## Key considerations 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). 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