My experience of 13 months of using the Anki spaced repetition memory system (me, in email). Makes memorization a choice, not a happenstancepic.twitter.com/Qwh25Rz4ux
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I couldn't find convincing published research to decide so in the end I made up my own simple spacing rules and was happy.
Duolingo has published some interesting stuff on using ML to estimate spacing. There are some obvious improvements, still...
If I was doing research in this area I'd look at pairs of similar flashcards (eg. see http://www.crwflags.com/fotw/flags/xf-like.html …)...
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