The simple act of taking a test helps you remember everything you've learned, even if it's not on the test, new U.S. research suggests.

In a study of 84 undergraduate students, psychologists at Washington University in St. Louis found that untested students recalled significantly less of what they had studied — even after having extra time to go over the material.

Researchers argue in the November issue of the Journal of Experimental Psychology that tests are more than efficient scoring tools. They are a "powerful memory enhancer."

Other studies have shown that testing helps students remember the material on which they have been tested, but it hadn't been clear whether typical mid-term and final exams were affected.

In the first study, 84 undergraduates were given 25 minutes to study a long, factual article about the biological characteristics and living habits of the toucan, a bird.

The students were then divided into three equal groups.

Those in the testing group were asked 22 questions about the material. Students in an "extra study" group read 22 additional statements about toucans, but did not take a test. Students in a control group were immediately dismissed.

The next day, all 84 students took a final test with 44 questions: the original 22 questions on which the first group of students had been tested, and an additional 22 questions.

Test simulated 'cramming'

Lead author and doctoral student Jason Chan said his research team used a 24-hour interval to simulate the way most students cram the day before a test.

On the new questions, students who answered the original 22 questions on the first day significantly outperformed both the students who had received extra study and the students in the control group.

The researchers concluded that testing, not extra study, accounted for improved performance.

The results "imply that as long as students have retrieved a concept, other related concepts should also receive a boost," Chan said.

The authors write that their results highlight the importance of testing, as opposed to re-studying material.