Who invented spaced repetition?

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Martin Mich, USA, asked: Who invented spaced repetition? Not Ebbinghaus?


Spaced repetition is a common intuition. It has been written about for centuries by various authors. The credit for computing optimum learning intervals goes to Dr Wozniak. The credit for the computational definition of the optimization procedures goes to SuperMemo. The term "spaced repetition" was popularized by SuperMemo and other applications that followed (e.g. Mnemosyne, Anki, etc.).

Answer 1


Nobody should ever take credit for discovering the spaced repetition. This is because of the fact this concept is pretty intuitive, and will be re-discovered in the mind of every attentive student sooner or later. In fact, we asked teenagers a set of questions about how their memory works. A large proportion can come with pretty good guesses without ever making any measurements. In particular they often correctly guess that the first optimum inter-repetition interval might be a week long, and that successive intervals will increase. Moreover, many could guess that the second interval might be a month long, and that successive intervals might double. In other words, spaced repetition is a common intuition.


Many authors over the centuries published their own guesses and intuitions about optimum intervals in learning. In particular, practitioners Cecil Alec Mace (1932), Paul Pimsler, and Tony Buzan managed to popularize their own variants of spaced repetition (with a fixed timetable, as in SuperMemo on paper). An oft mentioned Sebastian Leitner system was rather a flashcard prioritization procedure that did not produce a specific interval scheme, but could still be used to improve retention in learning. Those early recommendations were relatively popular in privy circles of memory artists and language students. However, they still remained largely unknown to the general population for years to come. The first specific spaced repetition algorithm was described in Wozniak, P.A., Gorzelañczyk, E.J., 1992, Optimal scheduling of repetitions in paired-associate learning. Acta Neurobiologiae Experimentalis, Vol. 52, p. 189 and Wozniak, P.A., Gorzelañczyk, E.J., 1994, Optimization of repetition spacing in the practice of learning. Acta Neurobiologiae Experimentalis, Vol. 54, p. 59-62 (PDF).

Note the evolution of terminology used by SuperMemo: (1) scheduling of repetitions (1992), (2) repetition spacing (1994), (3) spacing of repetitions (1995), and spaced repetition (1999).


SuperMemo can take credit for pioneering computational spaced repetition. First experiments that made it possible to compute optimum intervals were made by a student of biology, Piotr Wozniak in 1984-1985. Those were later computed more precisely with his software that evolved into SuperMemo (1987). At the same time, we heard of similar efforts by other researchers and programmers using fuzzy logic or set theory, but have no confirmation if these attempts were successful. Since then, we have only heard from a user of SuperMemo, Dawid Calinski from Poland, that his Full Recall program, based on neural networks, was also able to compute a good approximation of optimum intervals (2003). Those results have probably not been published yet. In the 1990s we heard of many attempts to investigate or implement spaced repetition that were abandoned upon the encounter with SuperMemo. In the new millennium, we see a transition from experimentation to implementation. We hear of many programs implementing SuperMemo algorithms, while very little of other experiments using various computational methods to replicate SuperMemo results.

To illustrate the omnipresence of spaced repetition see if this "Harvard patent story" does not resemble "SuperMemo on paper, 20 years later": http://harvardmagazine.com/2009/11/spaced-education-boosts-learning


In the early days of SuperMemo World, we actively searched for a well-recognized scientific term to use when referring to "the SuperMemo method" in scientific contexts. The company's marketing strategy was to move away from a "program developed by a student" to a "program based on a scientific method". Unfortunately, memory and learning literature was scant on publications other than short-term studies of the spacing effect (with a notable exception of H Bahrick's research on the retention of Spanish vocabulary). In the end, we moved from our own term "repetition scheduling" to a rarely used term "repetition spacing" (1994) that was later replaced with equally obscure and rarely used "spaced repetition" (Feb 1999). Over time, the term "spaced repetition" became more and more popular. In that light, SuperMemo can also take credit for making this generic scientific term take root in public's mind with a specific association to optimum intervals in learning.

See also: How was the term spaced repetition picked?


For the sake of computing optimum intervals in learning, we need a definition of the optimum interval. This definition was provided by SuperMemo only in 1991 with the introduction of the concept of the forgetting index. No other definition has been proposed and accepted until now even though different criteria might also be used for defining optimum intervals (e.g. with a focus on speed of learning rather than the level of retention).


Spaced repetition

A popular myth about spaced repetition was initiated by our file on the history of SuperMemo. Our statements on the contribution of Hermann Ebbinghaus to the experimental study of memory was misunderstood as "Ebbinghaus invented spaced repetition". Ironically, the distorted version found its place even in official marketing materials distributed by SuperMemo World. In actuality, like many scholars before him, Ebbinghaus expressed his intuitions about spaced repetition in his ground-breaking work on memory: Memory: A Contribution to Experimental Psychology (1885). He stated that: "with any considerable number of repetitions a suitable distribution of them over a space of time is decidedly more advantageous than the massing of them at a single time" (source: http://psy.ed.asu.edu/~classics/Ebbinghaus/memory8.htm).

See also:

Forgetting curves

Ebbinghaus indeed investigated forgetting curves, however, the attribution of the formula for recall (Retrievability) is incorrect. Many texts (incl. Wikipedia), list R=exp(-t/S) which was proposed in our paper: Two components of long-term memory. The formula is easy to recognize as S stands for Stability, which was proposed in the said paper (1995).

Another popular misconception related to forgetting curves is that we lose 50% of information in mere minutes! All users of SuperMemo know that for well-formulated knowledge, it may take a few weeks to forget 50% of knowledge. Ebbinghaus simply worked with nonsense syllables that are far harder to remember than the date of developing the Newcomen engine.

See: Ebbinghaus forgetting curve error.


Spaced repetition is a common intuition. It has been written about for centuries by various authors. The credit for computing optimum learning intervals goes to Dr Wozniak. The credit for the computational definition of the optimization procedures goes to SuperMemo. So does the ultimate choice of the term "spaced repetition" to attach to the process. In the end, the credit for the rising popularity of spaced repetition goes to countless users, students, and program authors that continue learning, implementing new applications, and spreading the word about this amazing technique and associated technologies.

Answer 2

Spaced repetition is as old as learning, however, to give credit to SuperMemo, we use the term "computational spaced repetition". It is not enough to just space the learning. Computing specific intervals is important.

1985 - Spaced repetition: Wozniak runs two simple experiments on his own memory and finds out how review intervals affect the recall. He formulates his first repetition spacing algorithm that will later be known as SuperMemo. His learning with the new method begins on August 25, 1985. This day marks the birth of computational spaced repetition, i.e. the technique in which knowledge is reviewed in optimum intervals that are determined by a computer with the goal of reaching a desired level of knowledge retention.

For more see: http://super-memory.com/english/history.htm

Ebbinghaus, like many others, spoke of the need to space the review. He did not propose any specific review scheme, and his measurements focus on short spans of time (hours, days, and weeks, rather than years).

See also