E-Factor

E-Factor, in Algorithm SM-2 in SuperMemo 1.0 for DOS (1987) and in SuperMemo 2.0 (1988), was a single number that stood for optimum factor, stability increase, and item difficulty. The number was increased as a result of good grades, and decreased after bad grades. Introduction of E-Factors in 1987 made it possible to split pages of questions into individual items/flashcards that can be handled individually in the learning process.

In 1995, E-Factors have been replaced with A-Factors that provided an absolute measure of item difficulty.

Algorithm SM-17 does not use E-factors or A-factors. Instead, difficulty is obtained by matching item's repetition history to the stability increase matrix to identify difficulty level that provides the best fit in performance.

Algorithm SM-18 is a step back in that difficulty is modified in successive repetitions in proportion to missed expectations. This comes from the realization that absolute difficulty is nearly never achieved in real life.

This glossary entry is used to explain "History of spaced repetition" by Piotr Wozniak (June 2018)