# OF matrix

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**Matrix of O-Factors** (or OF matrix) is a matrix of optimum factors in SuperMemo (starting with version 5 in 1989). The matrix is indexed by item difficulty (expressed as E-factor or A-factor), and by stability (expressed as repetition number or repetition number category).

In Algorithm SM-17, the role of the 2-dimensional OF matrix is played by the 3-dimensional stability increase matrix (*SInc[]*) with an added dimension of retrievability.

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