In the experiment of Cepeda et al. subjects can learn only two times. However, this is not realistic. You can learn over and over again. When you can learn repeatedly, how can you enhance your memory?

In 1972, a scientific journalist Sebastian Leitner proposed a learning method using a word card. It is called "Lightner System." The learning system uses word cards and boxes (*22). On the word card, you write a question on the front and the answer on the back. Prepare multiple boxes and set a review interval that gradually becomes longer, such as 1 day, 3 days, 7 days, 1 month, for each box.

When you create a new word card, you put it in the box with the shortest test interval. Cards that you can answer correctly goes to the next box with a longer test interval. The wrong-answered cards go to the box with the shortest test interval (Figure X). As a result of this process, you review cards that you do not remember at short intervals. You review cards that you already remember at long intervals. The review interval is automatically adjusted.

Fig: The correctly answered card goes the box with longer review interval. The wrongly answered card goes the box with the shortest review interval.

I have tried this system. I felt inconvenient to prepare boxes and kept the cards with the ring. However, it is a mistake. It is more inconvenient to take a card from the ring and put it to another ring than I expected. And the motivation disappeared.

You may want to do this system electronically. In 1987 Piotr Wozniak developed the first computer implementation of interval iteration, SuperMemo. The first version worked on MS-DOS, but now there are many implementations.

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Footnote *22: The initial Lightner system uses different sized boxes (1, 2, 5, 8, 14 cm), and you review the cards when the box is full.