Ebbinghaus Forgetting Curve
The Ebbinghaus forgetting curve refers to the decrease in the ability to memorize over time.
This was hypothesized by Herman Ebbinghaus, a German psychologist, who was also known to be the first to describe the learning curve. In 1885, Ebbinghaus suggested that learned information tends to be forgotten after days or weeks; however, such information will be easily remembered when reviewed. His studies also verified that memory goes down to 40% within the first few days and that the forgetting curve is exponential.
For instance, we all study algebra in high school. Later, unless you go into a field that requires everyday use of algebra skills, the rules for manipulating numbers and procedures that you learned easily slip away. However, it generally takes only a brief review of the rules to remember algebraic skills.