I read Thinking Fast and Slow and also I also read a better summary of the initial study, by the author I think. The 10,000 hour was taking from a study from top music performers (so, no random population), also there was a lot of variance; for ex among chess players some achieved mastery in 5,000 hours, others put in more than 10,000 and didn't achieve it. So lots of deliberate practice is good, arbitrary 10k hours is BS.