The research found that students who read a passage, then took a test asking them to recall what they had read, retained about 50 per cent more of the information a week later than students who used two other methods.

One of those methods—repeatedly studying the material—is familiar to legions of students who cram before exams. The other—having students draw detailed diagrams documenting what they are learning—is prized by many teachers because it forces students to make connections among facts. These other methods not only are popular, the researchers reported, they also seem to give students the illusion that they know material better than they do.

In the experiments, the students were asked to predict how much they would remember a week after using one of the methods to learn the material. Those who took the test after reading the passage predicted they would remember less than the other students predicted—but the results were just the opposite.

“I think that learning is all about retrieving, all about reconstructing our knowledge,” said the lead author Jeffrey Karpicke, an assistant professor of psychology at Purdue University. “I think that we are tapping into something fundamental about how the mind works when we talk about retrieval.”

The students who took the recall tests may “recognise some gaps in their knowledge,” said Marcia Linn, an education professor at the University of California, Berkeley, “and they might revisit the ideas in the back of their mind or the front of their mind.”

When they are later asked what they have learned, she went on, they can more easily “retrieve it and organise the knowledge that they have in a way that makes sense to them.”

The researchers engaged 200 college students in two experiments, assigning them to read several paragraphs about a scientific subject—how the digestive system works, for example, or the different types of vertebrate muscle tissue.

In the first experiment, the students were divided into four groups. One did nothing more than read the text for five minutes. Another studied the passage in four consecutive five-minute sessions. A third group engaged in “concept mapping,” in which, with the passage in front of them, they arranged information from the passage into a kind of diagram, writing details and ideas in hand-drawn bubbles and linking the bubbles in an organised way.

The final group took a “retrieval practice” test. Without the passage in front of them, they wrote what they remembered in a free-form essay for 10 minutes. Then they reread the passage and took another retrieval practice test.

A week later all four groups were given a short-answer test that assessed their ability to recall facts and draw logical conclusions based on the facts. The second experiment focused only on concept mapping and retrieval practice testing, with each student doing an exercise using each method. In this initial phase, researchers reported, students who made diagrams while consulting the passage included more detail than students asked to recall what they had just read in an essay.

But when they were evaluated a week later, the students in the testing group did much better than others. Why retrieval testing helps is still unknown. Perhaps it is because by remembering information we are organising it and creating cues and connections that our brains later recognise.

“When you’re retrieving something out of a computer’s memory, you don’t change anything—it’s simple playback,” said Robert Bjork, a psychologist at the University of California, Los Angeles.

“But when we use our memories by retrieving things, we change our access to that information,” Dr Bjork said. “What we recall becomes more recallable in the future. In a sense you are practicing what you are going to need to do later.”