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Who Gains When the Umpire Is a Machine? Evidence from the Automated Ball-Strike System (ABS)

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  • Seung Jin Cho
  • Jun Yeong Lee

Abstract

In 2024 the Korea Baseball Organization became the first top-tier league to replace the home-plate umpires ball-strike judgment with an automated system. Using over a million pitches from 2021 to 2026, we separate the change into the removal of human discretion and the imposition of a new zone geometry. The machine eliminated count-dependent calls, the value of catcher framing and a small advantage for star batters, rents worth only a few runs per player-season. Because the machines zone scales with height and treats every matchup alike, it redistributed strikes toward tall batters and across matchups, but few pitches are exposed to these changes, and their effects on plate-appearance outcomes are precisely estimated and close to zero. Pitchers re-optimized quickly by throwing higher, with left-handers catching up to an existing trend, whereas we find no evidence that batters adjusted. The results suggest that the incidence of replacing human judgment with a rule depends on its geometry and on exposure.

Suggested Citation

  • Seung Jin Cho & Jun Yeong Lee, 2026. "Who Gains When the Umpire Is a Machine? Evidence from the Automated Ball-Strike System (ABS)," Papers 2610.27576, arXiv.org.
    • Handle: RePEc:arx:papers:2610.27576
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    File URL: https://arxiv.org/pdf/2610.27576
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