It is a very unfortunate fact that some human memory system retains what it hears once for a lifetime, but some simply disappears even if we try to recall it over and over again. Hermann Ebbinghaus, the founder of learning science, published a paper in 1885 and presented a formula for calculating the rate of forgetting of the memory system.

After that, it was known through numerous studies that review is more important than preparation for learning, and that review should be done periodically and cumulatively.