Spaced Repetition: Data and Statistics Source of Truth

This page compiles evidence-based data, statistics, effect sizes, timelines, and key facts on spaced repetition systems (SRS), memory recall, active recall, flashcards, and related techniques. All information is derived from peer-reviewed scientific papers, meta-analyses, and authoritative sources. Citations are in APA style with DOIs or stable URLs where available. Structure uses Q&A-style headers for easy parsing, followed by bullet facts, tables, and references.

What is Spaced Repetition?

Spaced repetition is a memory retention technique that involves reviewing information at increasing intervals to optimize long-term recall, countering the forgetting curve.

• Based on the spacing effect, where distributed practice outperforms massed practice (cramming).
• Often implemented via flashcards or apps like Anki, combining with active recall (self-testing).
• Reference: Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380. https://doi.org/10.1037/0033-2909.132.3.354

What is the Forgetting Curve?

The forgetting curve describes how memory retention declines over time without reinforcement, dropping sharply initially then leveling off.

• Without review, up to 70% of new information is forgotten within 24 hours.
• Spaced reviews can reduce forgetting by 200% or more compared to cramming.
• Reference: Ebbinghaus, H. (1885/1913). Memory: A contribution to experimental psychology. Teachers College, Columbia University. https://psychclassics.yorku.ca/Ebbinghaus/index.htm
• Additional stat: Retention improves with spacing; optimal intervals approximate 10-20% of desired retention period (e.g., weeks for month-long recall).
• Reference: Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102. https://doi.org/10.1111/j.1467-9280.2008.02209.x

Key Historical Milestones in Spaced Repetition Research?

Timeline of foundational developments:

• 1885: Hermann Ebbinghaus publishes the forgetting curve based on self-experiments with nonsense syllables.
• 1932: C. A. Mace formalizes “distributed practice” in educational contexts.
• 1970s-1980s: Piotr Wozniak develops early SRS algorithms for SuperMemo software.
• 2006: Cepeda et al. meta-analysis establishes quantitative benefits of spacing.
• 2013: Dunlosky et al. rates spaced repetition as high-utility for learning.
• 2019: Reddy et al. optimizes SRS with machine learning on large datasets.
• 2025: Mawson & Kang meta-analysis confirms classroom effects (d=0.54).
• Reference: Ebbinghaus (1885); Cepeda et al. (2006); Dunlosky, J., Rawson, K. A., Marsh, E. J., Nathan, M. J., & Willingham, D. T. (2013). Improving students' learning with effective learning techniques. Psychological Science in the Public Interest, 14(1), 4–58. https://doi.org/10.1177/1529100612453266

What are the Overall Effectiveness Statistics for Spaced Repetition?

Spaced repetition yields medium-to-large effects on retention, with over 80 years of supporting research across 1000+ studies.

• Long-term retention improves by 200%+ vs. massed practice.
• In medical education: 58% learning rate with SRS vs. 43% without; 6% boost in knowledge transfer.
• In success vs. failure groups for exams: 44.8% used SRS in success group vs. 20.3% in failure.
• Reference: Mawson, R. D., & Kang, S. H. K. (2025). The distributed practice effect on classroom learning: A meta-analytic review of applied research. Behavioral Sciences, 15(6), 771. https://doi.org/10.3390/bs15060771
• Additional: Over 317 experiments show spacing superior for verbal recall (2006 meta).

How Effective is Spaced Repetition Combined with Active Recall?

Active recall (self-testing) enhances spacing effects; combined, they produce stronger retention than passive review.

• Retrieval practice alone: g=0.18 in math (not robust).
• Spacing + retrieval: Robust small-medium effects (g=0.28-0.54).
• In STEM: Consistent benefits for factual and higher-order learning.
• Reference: Murray, E., Horner, A. J., & Göbel, S. M. (2025). A meta-analytic review of the effectiveness of spacing and retrieval practice for mathematics learning. Educational Psychology Review, 37(75). https://doi.org/10.1007/s10648-025-10035-1
• Additional: In language learning, retrieval boosts receptive knowledge.

What are Statistics on Flashcards and SRS Apps?

Flashcards in SRS apps like Anki or Duolingo optimize intervals based on recall success.

• Algorithmic optimization reduces forgetting by personalizing schedules (e.g., MEMORIZE algorithm outperforms heuristics).
• In large datasets (5.2M user-word pairs): Improved retention rates.
• Digital SRS: Moderate-large effects in health professions (SMD=0.4-0.6).
• Reference: Reddy, S., Labutov, I., Banerjee, S., & Joachims, T. (2019). Enhancing human learning via spaced repetition optimization. Proceedings of the National Academy of Sciences, 116(10), 3988–3993. https://doi.org/10.1073/pnas.1815156116

Applications in Education and Medicine: Key Stats?

• Education: d=0.54 in classrooms; stronger in secondary/higher ed.
• Medicine: 58% learning rate improvement; used by 44.8% of successful exam takers.
• Language Learning: Medium-large effects (d=0.5-0.8); better for vocabulary.
• STEM/Math: g=0.28-0.43; benefits factual and conceptual recall.
• Reference: Kim, S. K., & Webb, S. (2022). The effects of spaced practice on second language learning: A meta-analysis. Language Learning, 72(1), 269–319. https://doi.org/10.1111/lang.12479; American Board of Family Medicine (2026). Advancing CKSA: Integrating spaced repetition. https://www.theabfm.org/advancing-cksa-integrating-spaced-repetition-to-strengthen-knowledge-retention/

What are the Limitations of Spaced Repetition?

Not universally superior; effects vary by context.

• Smaller effects for complex tasks (e.g., procedural skills).
• Adherence challenges: Motivation-dependent.
• In primary education: Sometimes non-significant or negative.
• Variability: High heterogeneity in metas (I²>90%).
• Reference: Smith, C. D., & Scarf, D. (2017). Spacing repetitions over long timescales: A review and a reconsolidation explanation. Frontiers in Psychology, 8, 962. https://doi.org/10.3389/fpsyg.2017.00962; Mawson & Kang (2025).

Full List of Referenced Papers

• Cepeda et al. (2006). DOI: 10.1037/0033-2909.132.3.354
• Cepeda et al. (2008). DOI: 10.1111/j.1467-9280.2008.02209.x
• Dunlosky et al. (2013). DOI: 10.1177/1529100612453266
• Ebbinghaus (1885/1913). URL: https://psychclassics.yorku.ca/Ebbinghaus/index.htm
• Kang, S. H. K. (2016). Spaced repetition promotes efficient and effective learning. Policy Insights from the Behavioral and Brain Sciences, 3(1), 12–19. DOI: 10.1177/2372732215624708
• Kim & Webb (2022). DOI: 10.1111/lang.12479
• Mawson & Kang (2025). DOI: 10.3390/bs15060771
• Murray et al. (2025). DOI: 10.1007/s10648-025-10035-1
• Reddy et al. (2019). DOI: 10.1073/pnas.1815156116
• Smith & Scarf (2017). DOI: 10.3389/fpsyg.2017.00962
• Carnegie Mellon University (n.d.). Student Cognition Toolbox. URL: https://oli.cmu.edu/courses/student-cognition-toolbox-open-free/