Self-grading vs. machine-grading in spaced repetition memory systems

In a Spaced repetition memory system, the key input to the control system is: did the user remember the prompt correctly? Some systems are self-graded: the user indicates whether they remembered or not. Others ask the user to input the answer; then the system computes whether the user was correct.

Self-graded systems outperform machine-graded systems in most respects.

Interestingly, Commercially-successful spaced repetition memory systems are all machine-graded