James: Measuring changes in quality of play based on hitters’ decline rates

From SABR member Bill James at Bill James Online on May 22, 2019:

I was trying to use hitter decline rates to evaluate the speed at which the quality of play in the majors is improving, and has improved over time; that was the theory. It didn’t exactly work. I had a twitter exchange with Mike Petriello in which I thought that he was making an unsupportable claim about major league players today doing things that players couldn’t do years ago, and in the middle of this discussion Voros offered the thought that if the quality of play in major league baseball has improved dramatically in recent years, it would be difficult to explain how the 40-year-old David Ortiz and the 38-year-old Adrian Beltre could perform at the levels they have. I thought “Oh, yeah, I ought to actually do that study”—”that study” being a study of the decline rates of hitters over time. In other words, almost all hitters are better at age 27 than they are at age 37, right? There are a few exceptions, hitters like David and Adrian and Nelson Cruz who just keep hitting as they age. But if the quality of play improves significantly in a short period of time, that should lead to an increase in the decline rate of aging hitters, relative to the league.

Hitters reach their peak at age 27, right? 26 to 28, let’s say. In measuring “decline” in hitting performance, then, we start at the peak, at ages 26 to 28. Referencing David Ortiz and Adrian Beltre, I decided to measure declines in ten-year increments. My study group, then, was all hitters in the years 1920 to 2008 who had 500 plate appearances in a season at age 26, 27 or 28. 1920, rather than 1919 or before, because the game before 1920 was just too different to be relevant to the study; 2008, because a ten-year decline rate cannot be established for a player from 2009 or later. The idea was that if hitters declined more between 1960 and 1950 than between 1940 and 1950, that would suggest that the improvement in the quality of play was greater in the 1950s than in the 1960s.

Read the full article here: https://www.billjamesonline.com/measuring_changes_in_the_quality_of_play_based_on_the_decline_rates_of_hitters/

Originally published: May 22, 2019. Last Updated: May 22, 2019.