Repetition history
Repetition history is a set of records representing repetitions in spaced review. In the minimum case, each repetition history record contains a pair: (1) repetition date, and (2) recall grade. SuperMemo records many other pieces of data in addition to that algorithmic minimum (e.g. the exact timing of repetition for sleep research).
This glossary entry is used to explain SuperMemo, a pioneer of spaced repetition software since 1987
Figure: Changes in memory status over time for an exemplary piece of knowledge. The horizontal axis represents time spanning the entire repetition history. The top panel shows retrievability (tenth power, R10, for easier analysis). Retrievability grid in gray is labelled by R=99%, R=98%, etc. The middle panel displays optimum intervals in navy. Repetition dates are marked by blue vertical lines and labelled in aqua. The end of the optimum interval where R crosses 90% line is marked by red vertical lines (only if intervals are longer than optimum intervals). The bottom panel visualizes stability (presented as
ln(S)/ln(days)for easier analysis). The graph shows that retrievability drops fast (exponentially) after early repetitions when stability is low, however, it only drops from 100% to 94% in long 10 years after the 7th review. All values are derived from an actual repetition history and the three component model of memory.