Was There a Structural Break in Barry Bonds's Bat?

In a recent paper, Fair (2008) utilizes monthly data on on-base percentage plus slugging percentage (OPS) to estimate the expected age profile of batting performance in Major League Baseball (MLB) and finds a peak performance at 27.6 years. However, he notes that a small group of 18 players have age-performance profiles that deviate significantly from the expected profile, most notably Barry Bonds. In this paper, we extend the work of Fair (2008) by investigating the time series properties of Bonds's OPS to test for a deterministic or stochastic trend and to search for structural breaks. While Bonds's performance is above average, we should not expect that deviations in his age-performance profile from that of the typical batter should contain a deterministic trend. In our investigation, we utilize unit root tests that estimate breaks using monthly data from 1986 to 2007. We find that Bonds's OPS deviations follow a deterministic trend with two structural breaks. In particular, we find that Bonds's OPS follows a positive trend to the age of 28.9 (June 1993), which coincides closely with the expected peak performance age. Following this, we find that Bonds's OPS was on a plateau until a second break in September 2000. At this point, at the age of 36.1, Bonds's OPS increases unexpectedly and then declines thereafter until his retirement in September 2007 at age 43.