Optimum interval
Optimum interval (in SuperMemo) is the ideal period of time to separate the review of individual pieces of knowledge. The optimum may be set to maximize the memory effect (i.e. stabilization). In practical applications, set retention of knowledge may be the chief optimization criterion. In most contexts, the term optimum interval implies an interval that results in the forgetting index of 10%. In newer SuperMemos, when the term stability is used in reference to intervals, it is also equivalent to the optimum interval at retrievability equal to 0.9. In SuperMemo 17 (and later), the first optimum interval is easy to find as it corresponds with the point where the first forgetting curve drops to the level of 90% recall. Intervals in SuperMemo are subject to random interval dispersion.
This glossary entry is used to explain "History of spaced repetition" by Piotr Wozniak (June 2018)
Figure: The first 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 interval for review at retrievability of 90% is 3.96 days. The forgetting curve can be described with the formula R=0.9906*power(interval,-0.07), where 0.9906 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)