The new RMSE is less likely to cheat and still easy to understand

https://github.com/ankitects/anki/commit/74a603e61bb4dfbb22c443635ffef6e42a3ae3c5

https://github.com/ankitects/anki/pull/3087

https://github.com/open-spaced-repetition/fsrs4anki/wiki/The-Metric