Triple-A Pitch Data Shows Robo Ump Strike Zone Didn’t Work As Hoped


Image credit: Triple-A Umpire Matt Bates Using The ABS System (Photo by Erica Denhoff/Icon Sportswire via Getty Images)

For years, many fans have expected that a fully automated ball-strike system taking over in the major leagues was just a matter of time.

However, with news this week that both Triple-A leagues will go to a challenge system for all games in the second half of the 2024 season, there’s a very strong indication that the full ABS system is unlikely to be headed to MLB any time soon, if ever.

It’s also another example of how MLB’s use of the minor leagues as a place to experiment on rules changes has proven useful for research and development.

MLB has been experimenting with full ABS systems for six seasons now, going back to its first testing in collaboration with the independent-turned-partner Atlantic League in 2019. There have also been experiments in the Florida State League and Arizona Fall League. But it was the introduction of the system to Triple-A in 2022 that seemed to indicate a fully automated ball-strike system may soon reach the majors.

There was only one problem: For as much as MLB has tweaked and experimented with the ABS system, the technology still does not fully replicate the strike zone as called by human umpires.

Some may call that a feature, not a bug. After all, if an always-watching roboump calls a perfectly consistent strike zone pitch after pitch, wouldn’t that be better than asking an imperfect human to do the same?

In reality, maybe not.

Which Triple-A Pitchers Have Been Most Affected By Robo Umps?

The ABS system isn’t perfect. So who was being hurt and who was being helped by automated balls and strikes?

The strikeout rate surged the first year Triple-A implemented the ABS system. The 19-inch strike zone set up for the ABS led to a larger number of strikeouts in ABS games compared to human umps.

They tweaked the strike zone again in 2023, reducing the width to 17 inches. The ABS zone was adjusted again in September, using Hawkeye to set the top and bottom of the strike zone based on a player’s stance, rather than a formulaic version of using a percentage of the player’s height to set the top and bottom of the zone.

This year, MLB went back to using a percentage of height to set the top and bottom of the zone, but deliberately increased the top of the zone by 2.5% compared to the 2023 zone to increase the number of strikes and reduce the number of walks.

It did help, but there is still a clear difference between the human and the automatic zone.

2023 Triple-A Games

Comparing 2023 Triple-A games using the ABS system (Tuesday through Thursday games) to challenge system games (Friday through Sunday games) showed significant differences. (The 36 Monday games were not studied).

Challenge Games62.40%10.45%23.87%2.80%4.73

That was a relatively dramatic difference, and one that produced multiple outcomes that aren’t part of MLB’s long-term goals. Yes, the strikeout rate dipped, but the increased walk rates and lower strike percentages were unwanted developments.

MLB’s tweaks to the ABS system had reduced the differences between the two systems in 2024. But those difference still remain.

2024 Triple-A Games

2024 Triple-A GamesStrike PCTBB PCTSO PCTHR PCTERA
Challenge System61.96%11.01%23.39%2.84%4.86

A look at a couple of teams gives examples of the differences between the two strike zones. The ABS zone is tighter, with very little ability to nibble on the edges of the zone.

Here’s what the Triple-A Durham pitching staff’s called strikes with the ABS zone looks like in 2024, as gathered by Synergy Sports.

Here are the Bulls pitchers’ called strikes with the challenge system, meaning the home plate umpired calls the vast majority of pitches. As is evident, pitchers can work the shadow zone and sometimes elevate a little above the zone for called strikes. If challenged, those pitches would likely be overturned, but they are part of the zone that hitters and pitchers generally expect to see.

Here’s the Las Vegas Aviator’s pitching staff’s called strikes with the ABS system

And here’s the Aviators’ called strikes with the challenge system.

We’ll follow this up with a look at which pitchers have been most helped and most hurt by the ABS system. Talking with pitchers and coaches, the ABS zone seems to be one that is better for “North-South” pitchers who work up and down in the zone and more difficult for pitchers who work “East-West” with pitches on the inner and outer edges of the plate, but we’ll be looking at how individual pitchers stats differ in a future post.

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