# R-Factor

**R-Factor** (*retention factor* or RF) is the value of U-factor for which the forgetting curve crosses recall level of 90%. The matrix of O-factors is a smoothed up equivalent of the matrix of R-factors. Both matrices correspond with a slice of the stability increase matrix for R=0.9.

R-Factors were first used in Algorithm SM-6 (1991), which was the first spaced repetition algorithm to collect forgetting curve data.

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

Figure:SuperMemo 7 for Windows (1994) displaying a forgetting curve approximated with an exponential function. Vertical axis represents recall in percent. Horizontal axis corresponds with time represented by U-Factor. With 21,000 repetition cases, the curve finally looks regular enough to hypothesize about the underlying cause of forgetting

Figure:An exemplary forgetting curve plotted by SuperMemo 8 in the course of repetitions (over 40,000 repetition cases recorded). R-Factor indicates that exponential regression points to the optimum interval of 7.146 days. This is then smoothed up via the OF matrix to 6.176 days. SuperMemo would use the first interval of 6 days in this case (if to ignore random dispersal)