The Inherent Umpire Bias
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News flash: being an umpire is hard. Their job is to judge whether a ball that’s capable of moving upwards of 100 MPH or breaking 25+ inches crossed through an imaginary zone before being caught. I don’t think many would argue that they have it easy, but every baseball fan would be lying if they claimed they’ve never been upset at a call, whether it’s your cleanup hitter getting rung up to end a rally or the opposing team’s leadoff hitter eking out a full-count walk on a pitch just inside. These calls have been in the game since its inception and will stay a part of the game while umpires still call balls and strikes. Finally being able to watch MLB games again got Tyler Zombro and I thinking about the trends in these calls and if umpires have an inherent bias to expand the strike zone for certain locations.
Methodology
The data set used was filtered down to only include pitches that were called strikes or balls as there’s no way of knowing what a pitch would have been called otherwise. A universal strike zone was then constructed using the PitchGrader model to help mitigate the height difference between batters. This provides a clear top (44.08" or 3.67' off the ground) and bottom (18.29" or 1.52' off the ground) to the zone while also giving a little leeway (1.47") off the edges of the plate since any part of the ball has to cross the strike zone to be a strike. The pitches were then filtered to be reasonably close to the strike zone to constrain wild misses.
Results
Left-Handed Hitters
Looking at a two-inch range off the plate against left-handed hitters shows that the outside pitches are called strikes at a 48% clip compared to a 21% clip inside. Increasing the range to five inches shows the same result: pitches off the plate away are called strikes much more often (26% away contrasted with 10% in). This disparity between outside and inside pitches shrinks when looking strictly at pitches from left-handed pitchers, giving credence to the idea that opposite-handed pitchers can live off the plate away much better.
Right-Handed Hitters
For right-handed batters, umpires are much more apt to expand the strike zone off the plate. Within the 2-inch window, 50% of outside pitches are called strikes while only 32% of those inside pitches are called strikes. Left-handed pitchers enjoy more success away within that zone with 54% of their pitches off the plate away being called strikes, backing up the idea that opposite-handed pitchers can live away better. In the 5-inch range, outside pitches are called strikes 28% of the time compared to 17% for inside pitches.
Overall
Umpires have a clear bias towards expanding the zone for outside pitches. While looking at pitches an inch outside/inside our zone, 58% of those outside pitches were called strikes compared to 36% of those inside. While both percentages are high for pitches that should probably be classified as balls, outside pitches were over 1.5 times more likely to be called strikes compared to inside pitches within the same range.
The difference in rate becomes even more pronounced the further off the plate you look. A range of two inches off the plate shows that outside pitches are 1.81 times more likely to be called strikes while increasing the range to 5 inches shows that outside pitches are almost twice as likely to be called strikes. The visual below showcases how much further off pitches away are called compared to those inside. It’s clear that umpires have called more strikes away and do so at a much higher rate.
I believe the main cause of this outside bias is that umpires set up between the catcher and batter, providing them a much better picture of the inside part of the plate. I believe this is also heightened with matchups of the same handedness as pitches on the inside part of the plate will approach from a straighter angle and therefore easier to judge its location.
With how much pitches move in today’s game, it’s not a surprise that more and more pitches are being called off of the plate. Pitchers can use this to their advantage and work away more often when they need to steal a strike. Getting two or three extra calls a game can be the difference between getting pulled early or putting together a quality start.
Further research is necessary to understand how and when the zone is expanded. Is it mainly in two-strike counts? Do fastballs or breaking balls get called strikes off the plate more? These questions and more will help us better understand what’s going on in the mind of an umpire… at least until the robots take over.
Thank you for reading. You can find more insights from Tyler and I at rdbaseballacademy.com. Please feel free to reach out to me on Twitter @k_camden for any further discussion or feedback.
