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. For a narrative walkthrough of the same literature, read I've read every major review on spaced repetition.
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).
| Meta-Analysis | Year | # Studies | Effect Size | Domain | Key Stat |
|---|---|---|---|---|---|
| Cepeda et al. | 2006 | 317 | Not specified (significant superiority) | Verbal recall | Optimal intervals increase with retention goals |
| Dunlosky et al. | 2013 | 10+ techniques reviewed | High utility rating | General education | Spacing rated highest for long-term retention |
| Kim & Webb | 2022 | 48 (98 effects) | Medium-large (d=0.5-0.8) | Second language | Stronger on delayed tests |
| Murray et al. | 2025 | 27 (53 effects) | g=0.28 overall | Mathematics | g=0.43 isolated; g=0.24 course-embedded |
| Mawson & Kang | 2025 | 22 (31 effects, N>3000) | d=0.54 (95% CI [0.31,0.77]) | Classroom learning | Larger with longer retention intervals |
How Effective is Spaced Repetition Combined with Active Recall?
Active recall (self-testing) enhances spacing effects; combined, they produce stronger retention than passive review. For practical technique guidance, see active recall with flashcards and our broader hub on flashcards for studying.
- 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. To put these findings into practice, build decks with our free online flashcard maker or follow a sustainable spaced repetition routine.
- 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/
Frequently Asked Questions
How much better is spaced repetition than cramming?
Long-term retention often improves by 200% or more versus massed practice. Meta-analyses (e.g., Cepeda et al., 2006; Mawson & Kang, 2025) show medium-to-large effects, with stronger gains when retention intervals are longer.
What effect sizes do meta-analyses report for spaced practice?
Classroom learning shows about d=0.54 (Mawson & Kang, 2025). Language learning metas report medium-large effects (d≈0.5–0.8). Mathematics spacing reviews report smaller overall effects (g≈0.28) that grow when spacing is isolated and well designed.
What is the optimal spacing interval?
A useful rule of thumb is that the interstudy interval should be roughly 10–20% of the desired retention period (Cepeda et al., 2008). Longer retention goals need longer gaps between reviews.
Does combining spacing with active recall improve results?
Yes. Retrieval practice during spaced sessions produces stronger retention than passive review. Spacing plus retrieval shows more robust effects than either technique alone across education, language, and STEM contexts.
Where does this page get its statistics?
All figures come from peer-reviewed papers and meta-analyses (Ebbinghaus, Cepeda, Dunlosky, Kim & Webb, Mawson & Kang, Murray et al., Reddy et al., and others), cited in APA style with DOIs or stable URLs.
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