Studies show that practice aimed at remedying weaknesses is a better predictor of expertise than raw number of hours; playing for fun and repeating what you already know is not necessarily the same as efficiently reaching a new level.
“Deliberate practice,” Ericsson declares sternly, “requires effort and is not inherently enjoyable.”
“the most notable differences between the practice sessions of the top-ranked pianists and the remaining participants,” Duke and his coauthors wrote, “are related to their handling of errors.”