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Thursday, September 09, 2004

Is Batting Average Luck?

One of the core sabermetric conclusions of the last 25 years is that batting average, officially adopted in 1876, is overrated. To reinforce that conclusions I today read Jim Albert’s study, A Batting Average: Does it Represent Ability or Luck?. In the study he was trying to determine what statistical categories could most be attributed to true hitter’s abilities and which were more the result of luck. He did this using both an empirical analysis by computing the season to season correlation in a number of areas and using the random effects model.

The statistics he measured included:

  • SO rate = SO / PA
  • IP HR rate = HR / Balls put in play (BIP)
  • BB rate = BB / PA
  • OBP
  • IP AVG = H / BIP
  • AVG
  • IP 2+3 AVG = (2B+3B) / BIP
  • IP S rate = Singles / BIP

What he found was that when ranked in order from more ability to more luck the statistics fell out into the order shown above. In other words a player’s strikeout, homerun, and walk rates are all more predictable and thus better indications of a hitter’s true ability than is batting average, which has a larger component of luck. It should be noted that his study looked at players who batted more than 100 times in the 2002 and 2003 seasons.

One of the things that immediately comes to mind is that perhaps the range of batting averages is smaller than the range of walk or strikeout rates because players who reach the majors have been pre-selected to fall within a restricted range. In other words, batting average may only appear as if it is less controlled by ability since the range of averages is smaller. I don't pretend to know enough statistics in order to answer that question.

The other interesting part of his study are the graphs he showed of these rates for Tony Gwynn and Barry Bonds over their careers. What is obvious from the graphs is that Barry Bonds’ career was following a fairly typical trajectory where he peaked around age 27-28 (1994-95) and began to decline until 1999 when suddenly his AVG increased, strikeout rate adeclined, walk rate increased, HR / BIP rate increased, singles / BIP rate increased, and doubles and triples / BIP increased dramatically and continue on those paths. These serve to highlight how extraordinary this last phase of Bonds’ career has been. Here's a quick graph I created that shows the trends.

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