Psychologist Cepeda and colleagues conducted experiments with 215 university students to space six months before the test. In this experiment, subjects input twice and test after six months of their second input. Cepeda changed the interval between the first and second inputs by 20 minutes, 1 day, 1 week, 1 month, 3 months, 6 months and compared the ratio of correct answers (*21).

The result is the highest at 55% correct answer rate when spaced for one month, roughly 40 to 50% for 1 week, 3 months, 6 months. The correct answer rate when spaced for 1 day and 20 minutes was 33% and 25%, which was far worse than when spaced the interval widely.

Fig: The relationship between the interval until the second learning and the correct answer rate

You may be worried that the content you studied completely disappear after 6 months since you studied. I thought so. The correct answer rate for the second input exceeds 90% in the case of 20 minutes or 1 day, while it is about 20% in the case of 6 months. In the former case, you feel "I can answer 90%. I remember them well." It is a good feeling. In the latter case, you feel "I forgot most of I learned 6 months ago." You feel sad. However, in the latter case, the result of the final test is better than the former.

It is similar to (3.4.3) Not confident but the score is high. Subjective feeling to remember does not match the objective results of tests. It is better to space intervals until you forget to train long-term memories.

In this paper, they conclude that it is better to review at the time of about one-tenth of the interval to the test when the interval to the test is more than 1 day. However, we can not always study on the optimal interval. If you stick to optimal intervals, you feel failed and decrease motivation. The correct answer ratio goes down gently after the peak. So even if you forgot to review after the optimal one-month interval and you reviewed it after 6 months, the results of the test are better than when you review it in a day. I recommend you not to worry too much about the optimal interval.

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Footnote *21: Cepeda, N.J., Coburn, N., Rohrer, D., Wixted, J.T., Mozer, M.C. and Pashler, H., 2009. Optimizing distributed practice: Theoretical analysis and practical implications. Experimental Psychology, 56(4), pp.236-246.