KBO’s automated strike zone changes the game: Are star hitters losing their special treatment?

The KBO League’s introduction of an automated ball-strike system in 2024 appears to have taken away more than human umpires’ discretion. It also coincided with a sharp change in how the league’s star hitters performed at the plate.

Prominent batters received fewer walks, struck out more often and saw their on-base percentages decline after the system, known as ABS or the “robot umpire,” came into use. A University of Michigan research team reported the findings in the international journal European Sport Management Quarterly in July.

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This suggests that favorable umpiring bias may have existed for famous hitters before ABS was introduced.

The study found that hitters with higher status experienced relatively greater deterioration in several measures closely linked to plate discipline than other batters. The researchers interpreted the pattern as evidence that a possible “star bias” in human umpiring had been reduced by automated decisions.

Song Jimin, a doctoral researcher in the University of Michigan’s School of Kinesiology and a co-author of the study, said the findings suggested that famous hitters may have received favorable calls before ABS was introduced. In particular, umpires may have been more likely to rule in their favor on pitches that caught the edge of the strike zone.

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Before ABS was introduced, when a famous player stepped into the batter’s box, umpires may have made more favorable calls on pitches that caught the edge of the strike zone.