This 10,000 hour theory has its origins in a 1993 study by Ericsson, where he looked at the performance ability of violinists, and showed that the playing ability was determined by the cumulative hours of training up to the age of 20. That is, the best experts had accumulated the magic number of 10,000 hours whereas those classified as merely "good" or "least accomplished" were found to have done only 8,000 or 5,000 hours of practice, respectively. The graph below illustrates this main finding, where yellow and orange are the best performing violinists. Clearly, the average time taken to get to the 'elite' level is 10,000 hours, at least when it comes to playing a musical instrument ...
Exceptions to the norm: What variance would indicate
There's another way to interpret this finding, which I'll get to later in the piece. First, a major statistical "omission" in the paper undermines how the conclusion of Ericsson and those who argue for 10,000 hours can be made.
I have that study, and what is remarkable about it is that Ericsson presents no indication of variance - there are no standard deviations, no maximums, minimums, or ranges. And so all we really know is that AVERAGE practice time influences performance, not whether the individual differences present might undermine that argument. Statistically, this is a crucial omission and it may undermine the 10,000 hour conclusion entirely.