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O-Factor (optimum factor; OF)
number which tells you how much intervals should increase to reach recall of 90%. O-Factors differ for different levels of memory stability and different item difficulty. They are normalized for the forgetting index of 10%. For the first repetition, i.e. there was no prior interval, O-Factor is assumed to be the same as the first interval (as if the prior interval was 1 day). For the first repetition, different O-Factors are computed for a different number of memory lapses. For example: if the O-Factor is 2.5, and the prior interval was 20 days, at repetition time, the new interval should be set to 2.5*20 days, i.e. 50 days.