What is ABS?
ABS stands for Automated Ball-Strike. It's a camera-and-radar system that tracks the actual location of every pitch as it crosses home plate. When a player thinks the umpire missed a call, they can challenge, and ABS reviews the pitch against the rulebook strike zone.
Unlike traditional replay review (managers, fair/foul, safe/out), ABS challenges are exclusively for ball-strike calls. The player who was directly affected has to call them in real time.
How a challenge works
Who can challenge
- The batter, if they think a called strike was actually a ball.
- The catcher, if a called ball should have been a strike.
- The pitcher, same as the catcher. Either can initiate.
The player signals the challenge themselves. No bench-coach assistance, no manager involvement. The signal is a quick tap of the helmet, hat, or chest protector. It has to happen immediately after the pitch, inside a hard window of a few seconds before the next pitch sequence begins.
Lost vs. retained challenges
Each team gets a fixed number of challenges per game. The mechanic that makes ABS interesting: if your challenge succeeds, you keep it. Lose a challenge (the umpire's call was confirmed) and you've burned one of your allotment. Win, and your count is unchanged. A team that consistently picks the right pitches to challenge can theoretically use the same challenge slot all game.
The two strike zones
The rulebook strike zone is defined as the area over home plate, from the midpoint between the shoulders and the top of the uniform pants, down to a point just below the kneecap. ABS sizes that zone per-batter, accounting for height and stance.
In practice, every umpire calls a slightly different zone. Some are pitcher-friendly (wider, lower), some are hitter-friendly (tighter, higher). That isn't a flaw in the people doing the job. Calling balls and strikes from behind the plate, two hundred times a game, while a 95-mph fastball moves laterally and a slider drops two feet, is genuinely hard. ABS doesn't care about reputation or pitch framing. It uses the strict rulebook zone, every time.
When a challenge happens, the system compares the actual pitch location against the zone. Inside the zone called a ball? Overturned to strike. Outside the zone called a strike? Overturned to ball. The magnitude of the miss, how far the pitch was from the zone edge, is what we track on every robbery card on the robberies page.
What our data shows
Overturned calls aren't marginal
The pitches that get overturned aren't close calls. The average overturned pitch this season was about 1.0 to 1.5 inches off the zone edge. The worst single calls landed at 3 inches or more, pitches well outside the zone that were called strikes. Those are the moments ABS exists to correct.
Roughly half of challenges succeed
Across MLB this season, the success rate sits around 53%. That's actually the right number. Players only challenge when they're confident, and confidence calibrates to roughly even odds. A 90% success rate would mean players are leaving free strikes on the table. A 20% rate would mean they're guessing. The roughly even line says players know what they saw, and most umpires are close enough that the disputed pitches genuinely could go either way.
Some zones diverge more than others
The umpires who get challenged most often aren't necessarily the ones missing the most calls. They're often working in high-leverage games where every call gets scrutinized. The signal we actually care about is overturn rate. The umpires with the highest overturn rate are the ones whose called zones diverge most from the rulebook. That's a measurable difference, not a character flaw, and it's the lens we use on the umpires page.
Why this matters
The strike zone has been the most subjective part of baseball for over a century. Two umpires calling the same pitch differently has decided games, careers, and championships. ABS doesn't replace the umpire. It gives players a way to overturn the worst misses while keeping the human element of the game intact.
What gets tracked here on Abstap is the gap between the called zone and the rulebook zone. Every robbery card is one moment of that gap, frozen and measured.
Explore the data
- The Robberies leaderboard. Every overturned call, ranked by leverage and magnitude.
- Umpires by overturn rate. Who's calling the rulebook zone, and who's calling something different.
- Team challenge stats. Which teams challenge most, and which win most.