Umpire Performance Improvements in MLB and Impacts on Run Scoring

• Activity Number: 513
• Type: Invited
• Date/Time: Wednesday, August 12, 2015 : 10:30 AM to 12:20 PM
• Sponsor: Section on Statistics in Sports
• Author(s): Brian Mills*
• Companies: University of Florida
• Keywords: Sports ; Baseball ; Umpires ; Generalized Additive Models ; Spatial Analysis ; Performance Evaluation

Abstract

New steroid policies are often credited as the cause for decreases in offense after their implementation in Major League Baseball. However, a number of other changes have taken place in MLB that have likely contributed to this decline. This work investigates one of those changes by leveraging data from Sportvision's Pitch f/x ball tracking system to analyze changes in strike zones called by MLB umpires from 2007 through 2013. I find incremental improvements in accuracy of umpire calls based on the MLB Rulebook strike definition, leading to a net growth in the size of the strike zone. These changes coincide with increases in umpire monitoring and training that began near the implementation of new steroid policies in MLB. The rate of improvement in accuracy is larger for younger umpires, which actually enter the league with lower accuracy rates than their more experience counterparts. I expand on the implications of these changes on game play, finding that umpire strike zone improvements are responsible for approximately 40 percent of the decrease in run scoring across the league.