A 2015 replication of Ebbinghaus's forgetting curve shows how fast new material fades, and a 1,354-person study shows when a review brings it back.

In a classic experiment, students who quizzed themselves recalled 61 percent a week later. Students who reread four times recalled 40 percent.

A 2015 replication of the forgetting curve shows how fast an intention fades in the first hour, and why writing the when, not just the what, is what works.