Learning with spaced repetition (SRS) is a standard way to memorize and retain information.
https://en.wikipedia.org/wiki/Spaced_repetition
Our fist version of SRS for goproblems uses a Leitner system (https://en.wikipedia.org/wiki/Leitner_system). Essentially missed problems get put into buckets for review, and as you get these problems correct they get moved into less frequent buckets.
As you get problem wrong as part of normal site usage, they will automatically get added to the training system and become available for review. If you want to remove a problem from training, select the Skip button during training, and you can choose to skip for just this session or forever.
We currently do not add problems missed during Rush mode or Time Trials to the training system, due to the time pressure element.