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Aging and slugging in baseball

The SF Chronicle asked Jim Albert, Professor of Mathematics and Statistics at Bowling Green, to analyze the relation between aging and slugging. JC at Sabernomics has been at this recently as well. At issue is this chart, which depicts the "home run rate" by age for famous sluggers.

The home run rate measures the % of balls in play that are home runs, which Albert argues removes random factors from performance. By this measure, McGwire and Bonds clearly peak at a later age than their predecessors. What's up? Albert is circumspect in the story, stating merely that "something is driving these changes." John Perricone at OBM - ace Bonds defender - pulls part of the answer out of a Peter Gammons story:

For history's sake, the home run rate in Major League Baseball from 1954 through 1976 -- essentially the careers of Willie Mays and Henry Aaron -- was one homer per 44 at-bats. The rate from 1993, when Barry Bonds first hit 43 homers, through this week was one homer per 32 at-bats.

So part of the Bonds-McGwire uptick late in their career can be attributed to a league-wide increase in the propensity to hit home runs, coincident with their reaching an advanced age. But this observation gives too little credit to Bonds and McGwire - compare the dark and light lines in the charts. These guys lapped the field - their contemporaries - at ages when the performance of their predecessors declined. Professor Albert is right: something's going on, and I'd wager much of it is benign. While it would be great to talk about these feats without reference to the S-word, unfortunately, it ain't gonna happen.