What did psychologist K. Anders Ericsson discover about the most successful violinists?

Gladwell claims that Ericsson discovered the most successful violinists were those who' spent at least 10,000 hours practicing by the time they turned twenty. Note, since Outliers was published, Ericsson and other researched involved in the project noted that the top violinists, as well as pianists, were those who spent the most time practicing alone.