Studien:
https://www.science.org/doi/full/10.1126/science.1199327
https://www.semanticscholar.org/paper/Repeated-testing-produces-superior-transfer-of-to-Butler/f0e926efa384f3c5a12071cfe393ce6c59d881d7?p2df
https://www.gwern.net/docs/spaced-repetition/1939-spitzer.pdf