The idea behind mature cards

I see in the manual that mature cards are defined with

Mature: A mature card is one that has an interval of 21 days or greater.

I was just wondering. Does anybody know how they came up with this interval of 21 days? Is there any reasoning behind it, or did they just think 3 weeks feels right?

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Perhaps based on Spaced Repetition learning steps?
If continuously answering cards correctly(Good), the steps would be 1day - 3days - 8days - 20days - 50days.

So, why not 50?

I would think there is some type of biological mechanism for when memories solidify and maybe some research that goes along with that regarding time frames, but I’m just speculating. I just thought maybe that’s where the number came frome.

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If the goal is to mature the card, students who study 50 days before the exam are very rare in my experience.

I don’t think exam preparation should be guiding design decisions of Anki too much, even though a notable number of users fit in that category. It should be possible to make design decisions targeted more generally towards learning via spaced repetition which would serve all users.

But again we still don’t know what made them choose 21 days anyways.

Many of the factors and concepts used in Anki are based on SuperMemo’s research, but Anki’s mature seems to be not in SuperMemo’s document. Continued Spaced Repetition will gradually stabilize memory retention, but there does not seem to be a magical number of days when retention increases dramatically, so I think it is arbitrary which date is determined as maturity.

With the default settings, by the time a card is answered after a 21 day interval, it will have been approximately a month since it was first introduced, and by that time cards tend not to be forgotten as frequently as younger cards. Young->mature is a gradual change though, and there’s nothing special about 21 days in particular.

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