The Interleague Advantage: A Difference in Differences Analysis

It has been argued that the introduction of interleague play in Major League Baseball provides an advantage to American League teams due to their use of the designated hitter. This paper examines whether this advantage actually exists and if so how large any advantage may be. The question is analyzed using a difference in differences model based on player performance data on interleague games from 1997 to 2008. It is shown that American League teams do have a small and statistically significant offensive advantage during interleague play. American League teams are estimated to have a 1.1 to 7.3 point advantage in batting average, a 0.1 to 9.8 point advantage in on base percentage, and a 1.2 to 9.9 point advantage in slugging percentage.