# Glossary:R-Factor

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- R-Factor (retention factor; RF)
- the number which says at which U-Factor (i.e. the measure of interval increase) the measured forgetting index (i.e. the measure of forgetting) is approximated to be 10%. This number is unique for different item difficulties and for different repetition categories (i.e. the measure of memory stability).
**R-Factors**can be seen as a vertical green line on forgetting curve graphs in SuperMemo