A novel computer-based system designed to declare balls and strikes is undergoing a trial in Major League Baseball (MLB) during their spring training exhibition games. This step follows a four-year testing period in minor leagues and is seen as a progressive step for the sport. The Automated Ball-Strike System (ABS) has won the support of Baseball Commissioner Rob Manfred, who believes the technology could provide valuable aid to MLB’s home plate umpires by as early as 2026, although it is not intended to replace human umpires entirely.
Starting in 2024, testing was primarily conducted on a challenge system wherein the human umpire is in charge of making the initial call. Feedback and data gathered from the current testing phase during spring training could lead to modifications in the ABS system for the forthcoming Triple-A season. The technology involves situating cameras around the stadiums that can monitor each pitch and adjudicate if it crossed home plate within the strike zone.
Initial rounds of testing saw umpires wearing earbuds, receiving the verdict of ‘ball’ or ‘strike’ into their ear, which they would then communicate to players and spectators through conventional hand signals. However, the introduction of the challenge system has brought a new dimension to this process. Under this system, during spring training, human umpires will continue to call every pitch, but now, teams have the opportunity to challenge two decisions per match, irrespective of extra innings.
Preserving a challenge is possible if a team’s reconsideration is successful, drawing parallels with the rules for video reviews in major leagues. Video reviews were first introduced for home run decisions in August 2008 and then extended to cover a broader range of appeals in the 2014 season. Nevertheless, the right to challenge a decision in the current format is limited to the batter, pitcher, or catcher, who must signal their intent by touching a helmet or cap.
Further, no advice from the sidelines is permitted, and a challenge must be mounted within a 2-second time frame. The diagram of the pitch and strike zone pertaining to the challenge will be publicly displayed on the scoreboard, as well as the broadcast feed. Following this, the umpire announces the updated score, a process that the MLB estimates to take an average of 17 seconds.
At present, the ABS system has been installed in 13 spring training parks, which serve as home grounds for 19 teams. These include Florida stadiums such as Detroit, Minnesota, the NY Mets, NY Yankees, Philadelphia, Pittsburgh, Toronto, and a shared ballpark between Miami and St. Louis. Furthermore, there are five test venues in Arizona that are also shared: the Diamondbacks/Colorado, Chicago White Sox/Los Angeles Dodgers, Cleveland/Cincinnati, Kansas City/Texas, and Seattle/San Diego.
Nearly 60% of spring training games are earmarked for the trial, however, the number of games each team might play under ABS testing can vary largely. The Diamondbacks, for example, are scheduled to participate in 29 ABS trial games, while the Cubs will only participate in seven. An advanced camera system known as the Hawk-Eye pose-tracking system is installed and used to monitor pitches, and ascertain if they are inside a strike zone. The strike zone is determined according to the height of each batter, measured without shoes prior to the first test match of the team.
It’s noteworthy that the time for calibrating the system has been estimated by MLB to take less than a minute per player. The majority of spring training parks have eight cameras installed for the test, with the exception of the Diamondbacks/Rockies stadium, where 12 cameras have been rigged. Interestingly, while the strike zone for big league umpires has an oval shape, the ABS system adheres to the rulebook and uses a rectangular strike zone.
The inception of testing ball/strike technology happened in the independent Atlantic League in 2019. The subsequent year witnessed a trial of the challenge system at eight out of the nine ballparks that constitute the Florida State League. The next step forward was taken in 2022, when ABS was introduced to five Triple-A parks, before it was expanded to all Triple-A stadiums in 2023. The initial model for the first three matches of each series used only the robot, with a human umpire adopting the challenge system for the final three matches.
This approach was also adopted at the start of 2024, until MLB transitioned to a purely challenge system halfway through the year. Over a complete Triple-A season, the total return rate was 51%, with 54% of challenges made by the defense leading to overturns, and 48% of challenges made by the offense leading to overturns. The average rate of challenges per game with a two-challenge limit in place was 3.9, including 2.2 by the offense.
Teams experienced a significant improvement in their success rate on video reviews last season, with an average of 53.7% successful challenges across all teams, and a high of 67.9% by the Boston Red Sox. A relatively small 1.6% of first pitches saw challenges, however, that statistic increased to 3.9% for two-strike pitches, 5.2% for three-ball pitches, and 8.2% for full counts. As games progressed, challenges were more likely to happen, with just 1.9% of pitches challenged in the first three innings, 2.5% from the fourth to sixth, 2.8% in the seventh and eighth and 3.6% in the ninth.
MLB has adjusted the dimensions for the ABS strike zone multiple times. The width started at 19 inches in 2022, reduced to 17 inches, which is consistent with the width of home plate. Also, the upper boundary of the strike zone was set at 51% of a batter’s height in 2022 and 2023, increased to 53.5% in 2024 in response to complaints from pitchers that it was too low. Since 2022, the bottom boundary of the strike zone has remained fixed at 27%, decreased from the initial 28%.
One of the key features of ABS is its unique ball/strike decision mechanism, where the decision is made at the midpoint of the plate, exactly 8.5 inches from the front and the back. This differs from the rulebook-defined zone considered by umpires to be a cube, declaring a strike as a pitch that bisects any part. In games where the ABS was fully deployed, MLB registered a slight uptick in strikeout rates by 0.5% and walk rates by 1%. The figures went up to 0.8% in challenge games.
Concerns were raised over the possibility that the broadcasted strike zone box could influence the decision to challenge, potentially leading to spectators coercing players to make a challenge. To address this, MLB is planning to explore several broadcasting alternatives, such as displaying only the box or the ball, or just the corners of the box.
iPad devices will be available in the dugouts for all teams, pre-loaded with an application named ProTABS. The application enables players to cross-verify pitches against their individual strike zones. Updates to the information will be made after each batting appearance, and players will have the option to provide feedback to MLB on each pitch and the overall system.