Open source algorithm for spaced repetition developed by piotr wozniak from the late 1980s. Used in anki.
Subsequent versions of this algorithm are not open source, and used in supermemo
See details here: https://www.supermemo.com/en/archives1990-2015/english/ol/sm2
EF = 2.5 I(1):=1 I(2):=6 for n>2: I(n):=I(n-1)*EF
For each correct response between 0 and 5 EF’:=EF+(0.1-(5-q)*(0.08+(5-q)*0.02)) Minimum EF = 1.3
{< youtube 1XaJjbCSXT0?t=1206 >}
Creator of supermemo and the related algorithms (such as sm-2).
Medium ( sm-2 default):