# O-factor

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**O-Factor** in SuperMemo version 5 through 16 is the optimum interval increase factor derived from the relevant forgetting curve. O-Factor determines how much optimum intervals should increase in review taken at optimum time. O-factors are grouped in a 2-dimensional matrix of O-Factors which is equivalent to a slice of the stability increase matrix with retrievability set to 0.9.

See also:

- Stability increase matrix (
*SInc[]*): an extension of the OF matrix into the dimension of retrievability - Introducing O-Factors to SuperMemo (1989)

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