Receiving tests itself strengthen your memory. In 1939, psychologist Spitzer did extensive memory experiments on 3,600 elementary 6th-grade students. Subjects read the science text only once on the first day, then take a test at intervals. Subjects are divided into several groups, and it is different when the test is performed for each group. By examining the difference in test results between the groups, we can see how the test affects the memory.

For example, if people take the first test on the seventh day, the correct answer rate is 31.5%. However, if people take the test on the day they read the material and receive the second test after seven days, the correct answer rate raises to 47.4%.

Fig: The correct answer rate changes depending on whether or not to test

Even if the time to test is long, the result rises. When people receive the first test after 28 days, the correct answer rate is 27.2%. However, when people receive the test 14 days after reading the material and receive the second test after 28 days, the correct answer rate has increased to 28.4%.

Those tests are unannounced. Subjects do not know that they will receive tests when they read the material. They are not told the result of tests or the correct answers. In addition, subjects can not read the text again after the test. It is impossible to review the mistaken problem after the test. Despite this, after receiving the tests, the correct answer ratio of the subsequent tests increased. The memories are strengthened by outputting, not by inputting again.

*17 Spitzer, H.F., 1939. Studies in retention. Journal of Educational Psychology, 30(9), p.641.