In statistics, the bell curve is a graphical representation of a normal data set. The center of the bell is the mean, which is average of all points in the data set. A standard deviation is how far data tends to be from the mean. In a normal data set, most (68%) data points lie within one standard deviation, the majority (95%) lie within two standard deviations, and almost all (99.7%) lie within three standard deviations of the mean.
Image Credit: University of Virginia
In scouting, player attributes are rated on a 20-80 or 2-8 scale, which is loosely based on the idea of a bell curve. In this case, our data set would be player skills at the major league level. For position players there are five commonly rated tools: hit, power, speed, defense, and arm. A 50 grade tool is considered to be average at the MLB level. Plus tools or 60 grade are above average. Double-plus tools are 70. And elite tools are 80. There is no grade above 80 because 80 is three standard deviations above the mean, and assuming a normal distribution very few data points will fall beyond three standard deviations. The same can be said of the opposite direction. 40 grades are below average. 30 grades are well below average. 20 grades are bottom of the scale.
Ultimately, the idea of the scouting scale following a normal distribution fails (hence the comment “loosely based”). It is difficult for players to succeed at the MLB level with 20 grade or 30 grade tools. The result is a disproportionately low number of 20 and 30 grade tools relative to 70 and 80 grade tools. Catcher speed ratings or weak-armed, rangy outfielders are a couple of examples, but they are more the exception than the rule.