A pop quiz may do more to help students retain information than an all-night session of cramming, according to U.S. psychologists studying the impact of learning methods.

The researchers found that testing is the key component of memory retention, far more important than studying after the material has been learned once.

Writing in Friday's issue of the U.S. journal Science, Purdue University researchers Jeffrey Karpicke and Henry Roederger described a series of tests for university students involving learned foreign-language vocabulary words.

In the experiments, college students first had to study Swahili and English word pairs and demonstrate the ability to correctly recall the pairs in a test.

Once that knowledge was demonstrated, the students were broken into four groups.

One group used a standard education method known to encourage better long-term memory: repeated studying and testing. Another group repeated studying without continual testing, while another repeatedly tested without studying. A fourth group did not study or take tests before the final evaluation.

While studying was necessary to first learn information, the researchers found that only repeated testing actually improved the retention of that knowledge.

Previous experiments have come to similar conclusions about the value of testing. But what was unique for the researchers was the finding that studying more contributed little to the process, they said.

"The experiment … shows a striking absence of any benefit of repeated studying once an item could be recalled from memory," they wrote.

"Although educators and psychologists often consider testing a neutral process that merely assesses the contents of memory, practising retrieval during tests produces more learning than additional encoding or study."

The researchers said further research is needed to assess whether the findings hold true for other subject matters, but they are confident the results will generally be the same.