Another missing point, and this happens frequently also in rigorous treatment-control studies, is the heterogeneity of responses to treatment. Say, 10k hours mi

Another missing point, and this happens frequently also in rigorous treatment-control studies, is the heterogeneity of responses to treatment. Say, 10k hours might be the average deliberate practice time needed to reach "expert level" (not true), but for some it can take 1k hours, for others 15k, for others infinite time (but they can drop off), and so on.