K. Anders Ericsson

Spent his career studying expert performance in chess players, musicians and athletes, and the central finding was not that practice makes perfect but that a specific kind does.

Deliberate practice means working at the edge of current ability, on a defined weakness, with immediate feedback and correction, which is nearly the opposite of enjoyable repetition.

His violinist study produced a number that Malcolm Gladwell turned into the ten thousand hour rule, and Ericsson objected publicly and repeatedly: ten thousand was an average, not a threshold, the quality of the hours was the whole point, and no amount of it guarantees anything. Peak is the book he wrote to correct the record.