MLB’s ABS Challenge System: What to Know for 2024

MLB Ushers in Automated Strike Zone Challenge System

Major League Baseball is entering a new era this season with the implementation of the Automated Ball-Strike (ABS) challenge system. While not a full transition to “robo-umps,” the system introduces a mechanism for players to appeal ball and strike calls made by human umpires, a change years in the making. The system, tested extensively in the minor leagues and during spring training, aims to improve the accuracy of strike zone calls without completely removing the human element from the game.

The arrival of the ABS challenge system has been met with cautious optimism. Yankees captain Aaron Judge, speaking to MLB.com last month, acknowledged the potential strangeness of the system for players. “I’m not an umpire. I’m a hitter. I’ve never been in the box trying to think about, ‘Is this a ball? Is that a strike?’ If I feel like I can hit it, I feel like it’s a strike,” Judge said, highlighting the inherent subjectivity of hitting and the challenge of evaluating pitches from the batter’s box.

How the System Works

At its core, the ABS challenge system grants each team two challenges per game to dispute ball or strike calls. Successful challenges are retained, theoretically allowing a team to challenge every pitch if consistently correct. However, challenges are limited to the hitter, pitcher, and catcher – dugout appeals will not be considered. The process begins with a player tapping their helmet to initiate a challenge, which must be done immediately after the pitch.

A key element of the system is the personalized strike zone. Each player’s strike zone is determined by measuring their height, with the top of the zone set at 53.5% and the bottom at 27.0%. While the width of the strike zone remains a standard 17 inches, the personalized zones account for individual player builds and stances. MLB opted for a two-dimensional strike zone over the middle of the plate after testing revealed that a three-dimensional zone resulted in an excessive number of strike calls, and a zone focused on the front of the plate also led to an inflated strike rate.

Once a challenge is initiated, the home plate umpire announces it, and the ABS result is displayed on stadium videoboards and television broadcasts within approximately 15 seconds. The goal, as with instant replay, is to correct the most egregious errors, though some close calls are inevitably expected to be reviewed.

Spring Training Insights

During spring training, teams averaged 4.5 challenges per game, a number likely to decrease during the regular season as players become more familiar with their personal strike zones and the system’s nuances. In Triple-A last season, the average was closer to four challenges per game, offering a more realistic expectation for the regular season.

Spring training data revealed some interesting trends. Catchers proved to be the most successful challengers, likely due to their vantage point, while pitchers were the least successful. Some teams in the minors even considered banning pitchers from challenging calls altogether, a strategy that could potentially be adopted by MLB teams. The challenge success rate in spring training was just above 50%, mirroring the accuracy rate of human umpires.

The data showed that batters challenged more frequently than catchers but were significantly less successful. This suggests that hitters may be more inclined to question calls, but catchers possess a better understanding of the strike zone and are more likely to have their challenges upheld.

Looking Ahead

The implementation of the ABS challenge system introduces a strategic element to the game. Teams will need to carefully consider when and who should initiate challenges, balancing the potential benefit of overturning a bad call with the risk of wasting a valuable challenge. Some situations may be deemed unsuitable for a challenge, such as a late-inning strikeout with minimal impact on the game.

There’s also evidence to suggest that challenging effectively is a skill. Data from Triple-A in 2025 showed a significant difference in success rates among players, indicating that some individuals are better at evaluating pitches and identifying incorrect calls than others. As teams gather more data and experience with the system, they will likely refine their strategies and identify players who excel at ABS challenges.

The ABS challenge system represents a significant step towards incorporating technology into baseball, aiming to improve accuracy and fairness while preserving the human element of the game. As the season progresses, it will be crucial to monitor how teams utilize the system, which players emerge as successful challengers, and whether the overall impact on the game is positive.