Background of the Robot Umpire Experiments
NPB's first official robot umpire experiment took place during the 2022 Phoenix League (instructional league) in autumn. TrackMan radar units were installed at stadiums in Miyazaki Prefecture, collecting pitch-tracking data on roughly 2,400 pitches across eight games. The experiment drew on insights from MLB's ABS (Automated Ball-Strike System) trials, which began in the independent Atlantic League in 2019, and was jointly managed by NPB's Technical Committee and Umpiring Department. Its two stated goals were verifying machine-call accuracy and visualizing umpire tendencies; results did not affect official game records. Additional tests were conducted in three Eastern League games in 2023, steadily building the data pool.
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Accuracy Verification Results
In the 2022 Phoenix League trial, overall agreement between TrackMan machine calls and the home-plate umpire reached approximately 94.2 percent. Analysis of the roughly 5.8 percent of pitches where calls diverged showed that 42 percent of disagreements occurred at the bottom edge of the zone (near the knees), followed by 28 percent on low-and-away pitches. Notably, on 0-2 counts (pitcher ahead) umpires tended to widen the zone slightly, dropping agreement to 91.3 percent, while on first pitches and 3-0 counts agreement rose to 96.8 percent. MLB's AAA tests reported similar patterns, numerically confirming that human umpires unconsciously adjust the zone based on the count. TrackMan's stated measurement error is within 1.27 cm, but pitches with heavy spin-induced movement, particularly vertical sliders and forkballs, showed deviations of up to 2.5 cm.
Player and Umpire Reactions
Players who participated in the trials offered mixed feedback. A young SoftBank Hawks pitcher noted that "losing strike calls on low balls outside the zone is tough, but if calls are consistent it's easier to plan around." Hitters were generally positive, with many saying that "if borderline takes are judged accurately, batters with good plate discipline will benefit." Umpires had more complex reactions. One veteran stated, "It doesn't negate the judgment skills built over years of experience, but it does require redefining our role." In its post-experiment report, NPB's Umpiring Department concluded that "machine calls should be used as an assistive tool for umpires, and final call authority should continue to rest with the umpire."
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Future Trial Plans and Remaining Challenges
NPB is considering full-scale ABS trial deployment in official minor-league games from 2025 onward. The specific plan calls for a "challenge system" test, allowing each team to request a machine review up to twice per game, across five Eastern League and five Western League games (ten total). On the technical side, measures to counter rain-related accuracy drops at outdoor stadiums, such as hydrophobic lens coatings and supplementary infrared cameras, are under review. Cost estimates for equipping all 12 farm-team home stadiums total roughly 400 million yen, and the split between NPB headquarters and individual clubs is a key negotiation point. With MLB eyeing major-league ABS deployment as early as the 2025 season, NPB faces growing pressure to align with international standards.