# Optimum factor

**Optimum factor** (also: optimal factor, O-Factor, OF) is a ratio of successive optimum intervals in spaced repetition. Optimum factors say how much intervals should increase in review. For example: if the O-Factor is 2.5, and the prior interval was 20 days, at repetition time, the new interval should be set to 2.5*20 days, i.e. 50 days.

Optimum factors depend on item difficulty and memory stability.

Optimum factors have been derived from E-factors in Algorithm SM-2 (1987-1988). While E-Factors combined the measure of difficulty (*D*) and stabilization (*SInc*), O-factors have been measured independently for all levels of difficulty (*D*) and stability (*S*).

Optimum factors have been introduced in Algorithm SM-5 (October 1989). As of SuperMemo 6, optimum factors have been normalized for retrievability of 90% (R=0.9). In Algorithm SM-17, optimum factors have been replaced with stabilization (*Stab90*) (see: Stabilization curve).

For details see: O-Factor (incl. O-factors for the first repetition).

See also: OF matrix.

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