April 24, 2017

James on Aging

Bill James looks at batter aging again, and comes up with a solution to some of the problems with his previous studies. As always with his work, it’s a fascinating read, and a great way to look at the numbers. One thing he now gets is that despite players in their late 30s have about the same averages as players in their mid 20s, the late 30s averages are actually worse:

We now know that the EXPECTED batting average, for this set of players, given the career batting average of each player in the sample and given the number of plate appearances that each player had, was .264. The .268 batting average that these players had at age 26 is four points OVER expectation.

The overall batting average of 26-year-olds in the study is .268, and the overall batting average of 37-year-olds in the study is .269. But the EXPECTED batting average of the 37-year-olds, given THAT set of players, was .280. The actual batting average of the group has increased from .268 to .269, but the expected batting average, considering who the players were, has increased from .264 to .280. This is what I did not know when I wrote about these issues in the 1980s. I knew that this increase had occurred, but I had no way of measuring it.

Well worth the read, and it’s free to the public.

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