Spaced repetition is a learning method based on computing optimum intervals that should separate repetitions of individual pieces of knowledge to ensure high retention. SuperMemo pioneered the use of optimization methods in spaced repetition. The most advanced version of spaced repetition has grown to become incremental reading.
See: Research background (1995).
Figure: A typical snapshot from an incremental reading process in SuperMemo. While learning about leptin, the student extracts important portions of the text, and marks keywords that might be used to form questions that will enhance memory in the long term. The questions are reviewed along a spaced repetition schedule. See this video example for explanation. The brain picture can be found here
Figure: The first review forgetting curve for newly learned knowledge collected with SuperMemo. Power approximation is used in this case due to the heterogeneity of the learning material freshly introduced in the learning process. Lack of separation by memory complexity results in superposition of exponential forgetting with different decay constants. On a semi-log graph, the power regression curve is logarithmic (in yellow), and appearing almost straight. The curve shows that in the presented case recall drops merely to 58% in four years, which can be explained by a high reuse of memorized knowledge in real life. The first optimum review interval for retrievability of 90% is 3.96 days. The forgetting curve can be described with the formula R=0.9907*power(interval,-0.07), where 0.9907 is the recall after one day, while -0.07 is the decay constant. In this is case, the formula yields 90% recall after 4 days. 80,399 repetition cases were used to plot the presented graph. Steeper drop in recall will occur if the material contains a higher proportion of difficult knowledge (esp. poorly formulated knowledge), or in new students with lesser mnemonic skills. Curve irregularity at intervals 15-20 comes from a smaller sample of repetitions (later interval categories on a log scale encompass a wider range of intervals)