Which Pitchers Should Fear Robot Umpires?

The future of baseball is filled with uncertainty these days, but there is one thing we do know about the future of the sport: robot umpires are coming!

Just ask Jayson Stark of the Athletic, who wrote back in January, “MLB is moving purposefully toward the world of electronic strike zones. And now that even the umpires’ union has pledged its cooperation, it’s almost a sure bet: This. Is. Happening.”

Stark’s not new to the robot revolution, as he’s written about the issue here and here as well. The fact is, electronic strike zones are already being implemented through trials in the Atlantic League and Spring Training. The fact that umpires have given their okay to begin testing makes their eventual implementation all but guaranteed.

Not only are electronic strike zones on their way, but it could happen sooner than you think. Stark suggests they’ll be in the majors potentially as early as 2022. Given that most prospects taken in the draft project to make their debuts in 3-5 years, teams are already drafting with the world of electronic strike zones firmly in their sights.

With this change coming, let’s table for a moment the many technical issues the league will face and instead consider the practical implications for the players. As I wrote after Stark’s report back in January, “The mental games used to inch the strike zone this way or that has long been a tool of the game’s best – from the hitters whose impeccable eye define it, to the pitchers’ whose pinpoint control push to expand it – but an automated zone will all but abolish the in-game politicking of the strike zone, giving hitters a new advantage they have long been without: certainty.” Per Statcast data, major league hitters swung at 33 pitches outside the zone per game in 2019 (~80,000 total for the year). It’s no surprise hitters struggle with zone control because the umpires themselves don’t always have a clear conception of where the zone lies. 

Roughly a quarter of all called third strikes in 2019 were on pitches that landed outside the zone. Home plate umpires made an average of 14 incorrect calls per game in 2019, which tracks with this Boston University study that looked at umpire accuracy going back more than ten seasons. The fact is, umpire error is making a huge impact on the game on a daily basis. The batter/pitcher relationship is the essential, critical matchup of the game. When this relationship loses integrity, the game itself suffers from existential crises. The whole reason umpires exist is to keep that crisis at bay. Most of us, after all, watch the game to see the talent of the batters and pitchers involved – not the umpires. Like it or not, when an umpire fails to properly adjudicate – when he misses a call – it muddies the waters of the game’s foundational competition.

Or in baseball terms, when ahead in the count, batters reached base at a .477 OBP clip in 2019. When behind in the count, that number drops to .209 OBP. That’s the difference between an absolute superstar and a sub-replacement-level hitter. When a batter falls behind because the umpire gifted a strike to the pitcher, the whole at-bat changes. The nature of the competition changes.

Digging into the data made available through Statcast, it’s not that difficult to find those incorrect calls. Build a book of umpire accuracy metrics for each pitcher by year, and we can get some clarity on how electronic strike zones are going to affect pitchers. The first question is this: are human umpires gifting more strikes to a particular kind of pitcher? To fireballers or workhorses or control artists or power pitchers?

If you’re interesting in walking through the data science behind this question, feel free to check out a video walkthrough of the process here, but the most interesting takeaway was this: there were four differentiating attributes of those pitchers who tended to get extra strikes versus those who did not: velocity, spin, role, and handedness.

Umpires tend to gift more strikes to pitchers with lower velocity, lower spin, to starters more than relievers, and to righties more than lefties. This makes sense if we think in terms of umpire vision. Given the active nature of a strike call versus the passive nature of a ball call, even a moment of uncertainty may lead an umpire to letting a strike go by without making a call. It makes sense, then, that umps might be more liberal with strike calls when they can see the ball more clearly. 

The league has trended towards higher velocity, higher spin pitchers in part because those pitches are more difficult for the batter to pick up, both because of their speed, and because the higher spin rate generally leads to a higher effective speed. It makes sense, then, that umpires would have difficulty picking up these pitches as well. Umpires may also have an itchier trigger finger with starters, whom they’re more comfortable with because they see them for longer periods of time, and with right-handers, who at least anecdotally, have less movement on their ball than southpaws.

Let’s look at some examples. Jon Lester has been one of the most consistently umpire-aided pitchers in the league. He runs counter to type by being left-handed, but those who’ve spent time watching Cubs games the last few seasons will tell you, Lester complains from pitch one and doesn’t stop griping until the ball is forcibly removed from his hand. He’s a bully, no doubt, and he bullies his way to extra strikes whenever possible. In 2019, roughly 25% of Lester’s called strikes were on pitches that landed outside the zone (versus ~17% average). He also had a very low percentage of “Stolen Strikes.” Of all the pitches he threw in 2019 at which the batter did not swing, only 2.4% of them were balls that should have been called strikes. 

On the other side, we find one of baseball’s premier villains: Aroldis Chapman. Chapman fits the mold of a guy that umpires aren’t likely to help out. He’s 99th percentile in fastball velocity, 92nd percentile in fastball spin, he’s a lefty, and he only pitches for usually an inning at a time at the highest-leverage moments of the game. Of all the pitches he threw in 2019 when the batter did not swing, he was “gifted a strike” on just 2.4% of those pitches, while 5.4% of those were “Stolen Strikes.”

There are other factors of course, beyond the velocity, spin, role, and handedness of the pitcher. Catcher framing certainly has an impact, and individual umpires themselves will have their own conscious or subconscious biases. But as we look ahead to a world of robot umpires, it does seem that lower velocity starters – workhorses and control artists – are going to lose the little bit of leeway that umpires are giving them now, whereas closers and firemen, guys with amazing pure stuff like Chapman will be even more valuable because they’re going to start getting some calls that umpires aren’t giving him now.