Why We Forget — and How Timely Review Helps Us Remember

Human memory does not work like a computer hard drive. Learning something once is usually not enough to remember it for the long term. Fortunately, memory and forgetting have been studied for more than a century, and one of the best-established learning principles is surprisingly simple: recall what you have learned repeatedly, increasing the intervals between reviews.

Hermann Ebbinghaus — measuring forgetting

The German psychologist Hermann Ebbinghaus (1850–1909) was among the first to investigate memory experimentally. In his 1885 book Über das Gedächtnis, he described experiments in which he memorised large numbers of meaningless syllables and measured how quickly he forgot them.

His work gave rise to the famous forgetting curve. Forgetting is rapid immediately after learning and slows down over time. His experiments also revealed an important principle: spreading learning and review across time works better than doing the same amount of work in one long study session.

Paul Pimsleur — review before you forget

Linguist and psychologist Paul Pimsleur (1927–1976) turned this principle into a practical learning system. In 1967, he published “A Memory Schedule” in The Modern Language Journal, describing Graduated Interval Recall.

The idea is simple: soon after learning something, we start to forget it. Recalling it before it is entirely forgotten strengthens the memory, allowing us to wait longer before the next review. A relatively small number of well-timed reviews can therefore produce lasting results.

Pimsleur also emphasised that one review schedule does not suit every word or fact. Intervals must be adapted to the material and the learner’s performance.

Sebastian Leitner — moving knowledge between boxes

In the 1970s, German science journalist Sebastian Leitner popularised a flashcard-learning method now known as the Leitner system. Cards are divided into several groups or “boxes”. A correctly answered card moves to the next box and is reviewed less often. An incorrectly answered card moves back for earlier review.

This focuses time and effort on difficult material rather than things already well known. The principle is a forerunner of many modern digital flashcard systems.

Remembering is learning

Simply rereading material is not enough for effective learning. Repeated reading can create a false sense of familiarity: the words look recognisable, but the answer may not come to mind without help.

Psychologists Henry Roediger and Jeffrey Karpicke studied this in experiments published in 2006. Some learners repeatedly reread material, while others had to retrieve it from memory. When tested after a longer delay, the learners who had practised active recall remembered more.

This is called the testing effect. A test does more than measure whether an answer is right or wrong — the effort of retrieving information strengthens memory.

That is why you should not turn over a flashcard immediately. Look first at the question, picture, sign or situation and try to answer. Even when recall takes a few difficult seconds, an important part of learning is happening.

Combining active recall with correctly timed reviews means that well-known material appears less often, while difficult questions return sooner. Finding an answer in your memory is itself part of learning.

What do larger studies tell us?

Also in 2006, Nicholas Cepeda and colleagues published a major analysis of spaced learning. They combined results from 317 experiments containing 839 comparisons of distributed learning.

The findings supported spaced repetition and identified an important pattern: the longer a fact must be retained, the longer the optimal interval between reviews may be. The question is therefore not only how many times to review, but also when.

The same principles guide SailorsDeck flashcards

SailorsDeck cards are not meant to make learners read the same information ten times in a row. First, a card presents a situation, sign, light, flag, fish or bird and asks the learner to think before seeing the answer: “Do I know what this is?” Successfully recalling it strengthens memory; an unknown answer returns for further learning.

In SailorsDeck’s digital learning system, the intervals for well-learned material gradually become longer, while difficult or incorrectly answered questions return sooner. The system brings together active recall, spaced repetition and increasingly long review intervals.

The aim is not to study more. The aim is to review at the right time.

Sources

Hermann Ebbinghaus (1885). Über das Gedächtnis: Untersuchungen zur experimentellen Psychologie.
Paul Pimsleur (1967). “A Memory Schedule.” The Modern Language Journal, 51(2), 73–75.
Henry L. Roediger III & Jeffrey D. Karpicke (2006). “Test-Enhanced Learning: Taking Memory Tests Improves Long-Term Retention.” Psychological Science, 17(3).
Nicholas J. Cepeda, Harold Pashler, Edward Vul, John T. Wixted & Doug Rohrer (2006). “Distributed Practice in Verbal Recall Tasks: A Review and Quantitative Synthesis.” Psychological Bulletin, 132(3), 354–380.