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Did you know ... spaced repetition was first applied using flashcards?

In 1972, a German science journalist built a system using a cardboard box and handwritten cards that changed the way we think about learning forever. Sebastian Leitner was a science communicator, and his method captured a truth that took researchers decades to fully explain: the timing of your review matters more than the hours you spend studying.

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Imagine studying something for three straight hours. The next day, you remember most of it. But a week later, barely 10% remains. Cognitive psychology research has demonstrated this pattern again and again since 1885.

In 1972, a German science journalist named Sebastian Leitner decided to do something about it. His solution was surprisingly simple: a cardboard box divided into five compartments and a deck of handwritten flashcards. That homemade invention became the first practical application of spaced repetition as a study technique, and its influence is still present in every modern learning application.

What is spaced repetition

Spaced repetition is a study method based on a straightforward principle: reviewing information at increasing intervals over time rather than cramming it all at once. Instead of spending an entire afternoon memorizing a topic, you review it multiple times across days or weeks, leaving a longer gap between each session.

The scientific foundation goes back to 1885, when German psychologist Hermann Ebbinghaus published Uber das Gedachtnis (On Memory), the first experimental study of how we forget. Ebbinghaus memorized lists of nonsense syllables and measured how much he retained after 20 minutes, one hour, one day, one week, and one month. He found that most forgetting happens in the first few hours: roughly 70% of new information vanishes within 24 hours without active review.

This pattern is known as the forgetting curve, and a modern replication by Murre and Dros (2015) (opens in new tab) confirmed that Ebbinghaus's findings still hold more than a century later.

Ebbinghaus also discovered the remedy: each review at the right moment flattens the forgetting curve, allowing information to be retained longer with less total effort. The key is studying when the timing is right.

The Leitner method: the first practical spaced repetition system

Sebastian Leitner (1919-1989) was a science journalist: someone who knew how to translate research into something practical. In 1972 he published So lernt man lernen (opens in new tab) (How to Learn to Learn), a book that became a bestseller and contained the first structured method for applying spaced repetition to daily study.

The Leitner system works with a box divided into five compartments and cards with a question on one side and the answer on the other:

  • Box 1: new or missed cards, reviewed daily
  • Box 2: reviewed every two days
  • Box 3: reviewed every four days
  • Box 4: reviewed weekly
  • Box 5: mastered material, reviewed once a month

The mechanics are elegant. When you answer correctly, the card advances to the next box, where it will wait longer before reappearing. When you get it wrong, the card returns to Box 1, regardless of where it was.

The result is that the cards you struggle with appear more often, and the ones you've mastered appear less. The system automatically prioritizes your weak spots without you having to decide. That is exactly what spaced repetition as a study technique needs to work: adapting to what you don't know.

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Why it works: the science behind the technique

The Leitner method rested on solid principles. Cognitive psychology has confirmed at least three of them through decades of research.

The spacing effect

Reviewing information at increasing intervals produces significantly better retention than studying it in a single intensive session. A study by Kornell (2009) (opens in new tab) demonstrated this directly with flashcards: 90% of participants learned more with spaced practice than with massed study. Here's the striking part: 72% of those same participants believed that massed study had worked better for them. Our intuition about how we learn frequently misleads us.

Retrieval practice

The act of trying to recall something, even if you fail, strengthens memory far more than simply rereading the material. The Leitner system forces you to actively retrieve the answer at each review. That effort of recall is precisely what anchors knowledge in long-term memory, a phenomenon researchers have named the generation effect.

Desirable difficulties

Robert Bjork, a researcher at UCLA (opens in new tab), coined the term desirable difficulties to describe study conditions that feel harder in the moment but produce more durable learning. Spacing is one of them: if a review feels too easy, it probably isn't doing much for your long-term memory. The difficulty of the effort signals that genuine learning is taking place.

"The timing of your review determines how much you retain long-term."

Examples of spaced repetition in practice

Spaced repetition is applied today across a wide range of contexts. Here are some concrete examples of how it is used for studying and maintaining professional competences:

  • Language learners: acquire vocabulary by reviewing difficult words daily and familiar words at decreasing frequency, automatically prioritizing what challenges them most
  • Medical students: memorize anatomy, pharmacology, and diagnostic criteria using apps like Anki (opens in new tab), which automate the Leitner system digitally
  • Pilots and aviation personnel: review emergency procedures and safety regulations periodically to keep their operational competence current
  • Financial professionals: keep their knowledge of regulations, compliance requirements, and complex products up to date in a constantly shifting regulatory environment
  • Healthcare workers: refresh clinical protocols, drug interactions, and patient safety procedures at optimized intervals

In all these cases, the principle is the same one Leitner applied with his cardboard box: what you don't know well appears more, what you've mastered appears less. The principle remains the same as in 1972. What has changed is the scale and precision with which the ideal review moment is calculated for each individual.

From cardboard flashcards to adaptive algorithms

Leitner's physical box worked well for a single person studying alone. But scaling the system to hundreds or thousands of professionals, each with different knowledge gaps, was impossible on paper.

Adaptive learning on a micro level takes Leitner's principle a step further. Drillster's algorithm determines for each person and each concept when the optimal moment to review has arrived, based on their individual response history. No two employees follow the same learning path, because no two people share the same gaps in knowledge and competences.

Completing a training course does not equal being competent. The forgetting curve does not distinguish between students and seasoned professionals. What sets apart organizations that maintain reliable competences is that they have stopped assuming a single training session is enough: knowledge and competences need continuous maintenance, exactly as Leitner demonstrated with his five-compartment box over half a century ago.

If you want to see how Drillster applies spaced repetition in your organization's context, discover how it works or explore how other organizations have tackled this challenge.

Want to make sure your team retains what they learn? Book a demo with our team and we'll show you how it works in practice.

References

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