D-backs to Play Most Games with ABS Challenge System in MLB

A primer with everything you need to know about the new automated balls and strikes challenge system MLB is trying out this spring

Jack Sommers | Feb 19, 2025

Jun 28, 2017; Phoenix, AZ, USA; Arizona Diamondbacks manager Torey Lovullo (left) argues with umpire D.J. Reyburn after being ejected from the game in the fourth inning at Chase Field. Mandatory Credit: Matt Kartozian-Imagn Images / Matt Kartozian-Imagn Images

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Major League Baseball is conducting a trial this spring, utilizing the Automated Balls and Strikes Challenge system. The Arizona Diamondbacks will play 29 spring training games utilizing the system, the most of any team in MLB.

MLB has been conducting trials and research for years, collecting data and making changes and tweaks. Speaking at MLB Media day in Phoenix on Tuesday, Executive Vice President of Baseball Operations Morgan Sword explained the system and the history.

ABS Challenge Rules

Each team will have two challenges and will retain successful challenges (i.e., those that overturn the umpire's call). There is no limit to the amount of successful challenges a team may receive, but once they have two overturned, they are out of challenges.

Only the batter, pitcher, or catcher can challenge - assistance from the dugout or elsewhere is not permitted and will result in the umpire denying the challenge.

Challenges must be made immediately after the umpire's call. The umpire has discretion, but this is expected to be enforced to within one to two seconds following the call.

What does it look like?

Fans in the stadiums and those viewing at home on television will see a graphic displayed when a pitch is challenged. The process from beginning to end took about 17 seconds on average in Triple-A last year, but less than that as the season went on. The challenging player taps his hat or helmet to indicate the challenge to the umpire.

Cactus and Grapefruit League

The five stadiums that are shared facilities in the Cactus League have the equipment installed. In addition to Salt River Fields (D-backs and Rockies), all games in Surprise (Rangers and Royals), Glendale (Dodgers and White Sox), Peoria (Padres and Mariners), and Goodyear (Reds and Guardians) will have the ABS system in effect.

Another eight stadiums in Florida, also home to Florida State League games, will also have games with ABS challenge system in place.

The average number of spring games per team throughout MLB that will use ABS is 21 in the Cactus League and 19 in the Grapefruit League. With 29 games in total, the D-backs' players will have a slight leg up in learning the system.

How is the strike zone determined?

One of the major issues MLB faced when implementing this system was how to determine the strike zone. What they settled on through trial and error was a zone that is the width of the plate, (17") and set back to the middle of the plate (8.5").

The top and bottom of the zone are determined by the player's height. The top line is set to 53.5% of the player's height, and the bottom is 27.5% of the player's height. MLB has devised a standardized method of measuring a player's height, which they already did last year for minor league players.

This spring they are going around team by team to take these standardized measurements of all major league players. From past work done in the minor leagues, we can expect that many player's true heights will end up being significantly shorter than currently listed.

Quite a number of D-backs players have already seen large corrections to their heights. For example Jorge Barossa's height was corrected from 69 to 65 inches.

That -4 change was the largest difference recorded. A.J. Vukovich went from 77 to 74 inches. Adrian Del Castillo lost two inches, going from 71 to 69, and Blaze Alexander went from 73 to 71.

Differences between human vs. ABS

Another aspect to consider is that with human umpires, there are differences to how the strike zone is called under different circumstances. It's a well known fact that the strike zone tends to expand with three balls and tighten with two strikes.

Beyond that however, umpires tend to call the high strike that is above the zone more often against shorter batters, and the low strike that is below the zone more often against taller hitters. Thus the biggest beneficiaries of a standardized zone are likely to be those players on the more extreme ends of height ranges.

Why not just go to a full ABS system?

MLB initially started with a full ABS system in their earliest trials in the minor leagues. However the overwhelming feedback from players and coaches was that was too much of an intrusion of technology into the game. It also isn't necessary.

Major League umpires get approximately 95% of calls correct. Of the 5% of miscalled pitches, only a small number come in high-leverage, meaningful at-bats.

The majority of players and coaches, 54% prefer the challenge system. 38% would choose to just keep it the way it is with human umpires making 100% of the calls. Just 8% were in favor of a full ABS system.

MLB's research and fan polling shows that 47% would prefer the challenge system, compared to 23% Full ABS, and 30% human.

How many challenges occur per game and how many are overturned?

On average teams challenged about four times per game in Triple-A last year. The split between successful and unsuccesful challenges was roughly 50/50.

The frequency of challenges nearly doubles from the early innings to the ninth inning. 1.9% of pitches were challenged in the first three innings, and 3.6% were challenged in the ninth inning.

Likewise, 3-2 counts were most likely to be challenged, 8.2% of the time, compared to under 3% in total for all other counts.

When will MLB institute the system in MLB games?

Sword indicated the earliest that might take place would be for the 2026 season but that's far from certain. They need to first collect all the information from the trials being conducted this spring, and get feedback from players, coaches and umpires.