Questions about the spaced repetition algorithm used in SuperMemo

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Huge interval discrepancies between Algorithm SM-15 and SM-17

Question

How can SM16 and SM17 differ by 800% in their estimation of the next optimum interval. I have made a repetition (cloze) and the discrepancy now stands at 7 vs. 61 days.

Answer

SuperMemo 16 assumes the current interval is optimum even if you set it by hand. SuperMemo 17 will look at the estimated stability, which depends on the entire repetition history. If you set the interval to 7 and stability is 61, the next optimum interval will be:

800% difference is not the worse you can see. If you take an item with stability 10,000 and give it an interval of 1 day (e.g. because of its importance or fear of forgetting), the difference in optimum next interval will be 1,000,000%. Once you decide on a new interval, SuperMemo 16 takes your word for it, SuperMemo 17 does not.

If you tend to pay attention to intervals, in SuperMemo 17 you may need to hold your nose and accept the new reality. If you care about high speed, high retention and efficiency, you may need to accept that 10% forgetting is still painful and holding it off manually contradicts the principles of efficient learning.