U-Factor
U-Factor (UF) - convenient measure of time for plotting forgetting curves in SuperMemo. U-Factor equals to the ratio of the current interval and the previously used interval. If the item has not been repeated yet, i.e. the previous interval is not defined, U-Factor equals the first interval. The greater the U-Factor the greater the increase of the interval between repetitions. By employing U-factor, SuperMemo makes it easy to compare forgetting curves for intervals measured in days or in decades.
For more see: Employing forgetting curves in spaced repetition (1991)
This glossary entry is used to explain "History of spaced repetition" by Piotr Wozniak (June 2018)
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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