Why do E-factors always equal 2.5 at the beginning of repetitions?
Historically, the value 2.5 comes from the development of SuperMemo; not from any specific property of memory. It could be 0.25 or 25 as soon as all the remaining optimization parameters were modified accordingly. You can look at E-factors as a very rough approximation of O-factors for the forgetting index equal to 15% and the repetition number greater than 2.
