In a now-famous experiment, (Ericsson) and his colleagues compared three groups of expert violinists at the elite Music Academy in West Berlin. The researchers asked the professors to divide the students into three groups: the "best violinists" who had the potential for careers as international soloists; the "good violinists"; and a third group training to be violin teachers rather than performers. Then they interviewed the musicians and asked them to keep detailed diaries of their time.
They found a striking difference among the groups. All three grounds spent the same amount of time participating in music-related activities. All three had similar classroom requirements making demands on their time. But the two best groups spent most of the music-related time practicing in solitude: 24.3 hours a week, or 3.5 hours a day, for the best group, compared with only 9.3 hours a week, or 1.3 hours a day, for the worst group.
-From "Quiet: The Power of Introverts in a World that can't Stop Talking" by Susan Cain