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From fangraphs
The invention of the scale is credited to Branch Rickey and whether he intended it or not, it mirrors various scientific scales. 50 is major league average, then each 10 point increment represents a standard deviation better or worse than average. In a normal distribution, three standard deviations in either direction should include 99.7% of your sample, so that’s why the scale is 20 to 80 rather than 0 and 100. That said, the distribution of tools isn’t a normal curve for every tool, but is somewhere close to that for most.
MLB average is 50 or 5 using a 2-8 scale. Normally only 1 player has an 80 rating in each category. For instance A. Judge might be the only player with an 80 rating for power.
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