We have known for several years that right-handed and left-handed batters do not see the same strike zone in the major leagues. The strike zone for left-handed batters shifts about two inches toward the outside. This observation goes back at least to Dr. John Walsh’s analysis of PITCHf/x strike zone data in 2007.

In my analysis of the strike zone earlier this year, I suggested the following:

For example, as mentioned earlier, the zone for left-handed batters is shifted toward the outside. Do umpires have some bias against left-handed hitters? If so, why? Perhaps a more likely explanation is that they simply call more strikes outside to lefty hitters because that’s where the catchers are setting their targets, and the umpires are using the target as a cue. While right-handed batters see 58 percent of pitches outside of the midpoint of the plate, left-handed batters see 66 percent of pitches on the outside half. The average pitch to a left-hander is 2.4 inches farther outside than the average pitch to a right-hander, which dovetails nicely with John Walsh’s finding that the average strike zone for a left-handed batter was shifted 2.3 inches farther outside than the average zone for right-handed batter.

Today, I want to investigate an alternative explanation, not because the above explanation is faulty or insufficient, but rather as part of the process of understanding the strike zone and how home plate umpiring works.

While researching hit batters, I observed that the umpire positioned himself differently on inside pitches to Chase Utley and Raul Ibanez. It turns out that changes in umpire positioning are not a rare event.

I decided to track the position of a home plate umpire’s head over multiple pitches. I began with the game from which I drew the original Utley-Ibanez comparison—Phillies at Cardinals, June 24, 2011, umpired by Mike Estabrook. I tracked the 27 pitches thrown in the first inning of that game, and here is what I observed. The horizontal dimension is from the umpire’s perspective.

Home plate umpires normally stand in the slot between the batter and catcher. When Estabrook does this, he is much closer to the plate with a left-handed batter up. With a lefty batting, he stands right in line with the edge of the plate. With a righty batting, he is about eight inches inside. In addition, when the catcher sets up for an outside pitch, Estabrook’s position is lower and usually a little farther outside.

I tracked the same data for two additional games in Busch Stadium, which has the advantage of a straight center field camera view. One game involved the Giants and Cardinals on June 2, 2011, umpired by Tim Welke. The other game involved the Blue Jays and Cardinals on June 24, 2011, umpired by Tim McClelland.

Welke’s position for left-handed batters is similar to Estabrook’s. However, he is much closer to the plate for right-handed batters, almost a mirror image of how he lines up for lefties.

McClelland’s technique was much different from the other two. Estabrook and Welke moved into position immediately after the catcher moved and were stationary prior to and during the entire flight of the pitch. McClelland started his move later and kept moving, following the pitch down lower in his stance, as well as laterally, all the way until it hit the catcher’s mitt. Moving during the pitch seems like a bad idea for consistent strike calls. Unfortunately, the facts get in the way of that theory. McClelland is one of the best-rated umpires according to the players and one of the best-rated home plate umpires according to my own measurements of percentage of correct calls. (Tim Welke is one of the worst-rated at strike calls, and Mike Estabrook is in the middle of the pack, though all the umpires are very close together in that measure of performance.)

Let’s look at the strike zones for each of these three umpires, first for right-handed batters, and then for left-handed batters, based upon pitch location data from 2010 and 2011. The black box shows the approximate left and right boundaries of the rulebook zone and the average top and bottom boundaries called by major-league umpires for a batter of average height. The perspective is from the umpire’s viewpoint.

You can see that McClelland does a good job of calling the rulebook zone left and right, while Welke and to some extent Estabrook have wider zones for right-handed batters.

All three of the umpires have zones shifted outside for left-handed batters, Welke the most, Estabrook in the middle, and McClelland the least. I chose McClelland and Welke because they represented umpires near the extremes in zones for left-handed batters (and because both had worked games behind the plate in St. Louis this year).

I had hoped that the variations in umpire positioning might explain the shifts in their strike zones. If so, it is not clear to me how, at least not from this sample of three umpires. Still, I found it interesting that umpires have predictable patterns of positioning based upon the positions of the catcher and batter.

I would like to touch on one other aspect of umpire strike zone evaluation. If you look closely at Estabrook’s plot for right-handed batters, you may notice something strange. Did he really call a strike on that pitch located at 2.96 feet high and 1.87 feet wide of the center of the plate? That is 14 inches off the edge of the plate!

If you believe the PITCHf/x data, then yes, he did, but it might be wiser to believe Estabrook than the data in this case. I reviewed the video of that pitch, and while it does appear to have been outside off the plate, it almost certainly was not 14 inches outside. By no means do I advocate video as an accurate method for judging strike calls. Relying on video leads to all sorts of problems, not least not being able to tell when the ball crossed the plate, even with a straight center-field camera view as we have for the pitch in question here. With those caveats, here is a still frame showing roughly where the ball crossed the plate.

From this angle, the pitch appears to be maybe five inches outside.

When I wrote earlier this year about the accuracy of PITCHf/x plate location data, I described a method for determining the error in the data. I happened to include Busch Stadium as the example, and you can see this game, the first of the 2010 season, as a prominent outlier.

I estimate that the PITCHf/x system was recording plate locations about four to five inches too far outside for the first two games of the 2010 season. That would make the pitch only nine inches off the plate, which still seems a bit much, both by the video and by the other strikes that Estabrook has called over the last two seasons. Perhaps the PITCHf/x plate location error was even bigger than I have estimated.

The PITCHf/x plate locations are generally within an inch or two of being accurate, but on rare occasions, they can be a lot worse. It is important to keep this fact in mind when evaluating umpires. Otherwise, you may find yourself thinking that Estabrook or some other umpire needs a seeing-eye dog on the field to assist with his zone.

The fact that the catcher target is set outside more often for left-handed batters may be the primary driver of the outside shift of the strike zone for left-handed batters. However, that would not seem to explain the bulk of the variation between umpires, who, over the course of several seasons, should see approximately the same fraction of left-handed batters and a diverse mix of pitchers. Variations in umpire positioning or other umpire mechanics would seem to be a natural explanation.

However, though I was able to identify unique positioning traits for the three umpires I investigated here, I could not correlate those variations to the variations in the umpires’ strike zones.

Thank you for reading

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Mike, thanks for sharing. Have you considered the impact of occular dominance on the variations? A succinct explanation is here:

It would be an interesting phone call to the umpires' union to see if they would contribute anything like this to a research project.
That's an interesting thought. It seems to me that if ocular dominance were the primary cause, we'd have basically two groups of umpires, one much bigger than the other. Instead, the distribution of left-right shift of the umpire strike zones roughly follows a normal distribution. Also, I would think that shifts in head position on the order of a foot would have a greater effect than dominance of eyes that are a few inches apart. But it is possible that ocular dominance is playing a role here, along with other factors, and I appreciate you bringing it up.
It seems to me that it would be better to set up a camera directly over top of homeplate. Even if a positioning camera directly over top isn't practical, some sort of downward looking camera seems like it would be much better than relying on cameras in centerfield in terms of being able to accurately judge if a ball crossed the plate. Ideally, you would have some sort of sensor in homeplate that could tell if a part of the ball crossed homeplate.

The vertical limits may be a bit harder to judge automatically since it isn't a fixed size, but it would be easier to judge if you had a reliable way for determining the horizontal position. Imagine if the umpire had a buzzer in his hand that was tied to an automated horizontal width judge that would activate the buzzer if the ball crossed the plate. Now all the umpire has to do is judge the vertical limits. The umpire would no longer have to compromise on his positioning. He could thenstand wherever gave him the best view of the vertical portion of the strikezone.
The two PITCHf/x tracking cameras are mounted so that they have a downward-looking angle. They are probably at about the optimum placement for that, unless you suspended a camera on a special mount above the playing field, where it would be subject to being hit by popups.

Even as such, the PITCHf/x cameras have some error in tracking the ball, as discussed in the article, though I believe there are techniques for significantly improving the calibration that MLBAM and Sportvision are not currently availing themselves of.
a special, *wind-resistant* mount, I would think ... lest you get "shaky" results
Lots of food for thought here. Very interesting to see that many more balls were called strikes than strikes were called balls. In other words, pitchers are getting the benefit of the doubt more often than hitters are, at least on the horizontal measures.
Meanwhile, pitchers are getting severely squeezed on the vertical, since no umpire in the past 10 (?) years has called a pitch above the belt a strike, despite the fact that "the STRIKE ZONE is that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants." Pitchers have lost a significant chunk of the strike zone.
I realize the Pitchf/x data is a bit iffy, but I wonder if you would get more granularity on this issue by breaking it down for RHP vs. LHP and/or by fastball/breaking ball/change.
Umpires call a few inches above the belt, but you are correct, they do not call up to the rulebook top of the zone, which would be somewhere around 3.8 feet.

I'm not sure this reshaping of the zone is necessarily a bad thing for the game, though it would of course be much better if the rulebook and reality were in sync with each other. Bruce Weber has a discussion of this topic in his excellent book on umpiring, "As They See 'Em", in which he talks about how umpires basically call the zone in the region where hitters can hit the ball. The rulebook zone, on the other hand, doesn't reflect that reality very well.

My point on bringing up PITCHf/x data calibration issues was not to cast aspersions on that data, though I do think it is a healthy reminder for people who want robot umps or the equivalent thereof. PITCHf/x data is still much more accurate than the judgment of a TV viewer, and most of the time it's probably also a more accurate judge of pitch location than the umpire.
Interesting point that you pass on from Weber, but don't you think that a consistently-called strike zone, as prescribed by the rulebook, would result in hitters making the adjustments so that the zone IS the region where hitters can hit the ball? Batting stances aren't cast in concrete, they're an adaptation to the strike zone as called.
I'm sure hitters would do fine with a smaller strike zone. Pitchers maybe not so much. I'm paraphrasing from memory here, but one of the umpires (or ex-umpires) interviewed in the Weber book said something like, "The bat is 32 or 34 inches long. If the batter can't cover a few inches off a 17-inch plate, he's doing something wrong."

A number of umpires also spoke about how they are urged by the league to "find strikes" and "hunt for strikes" in order to keep the games moving.
Handedness of the pitcher and pitch type do not have a large effect on the zone, not to the extent of the 2-3 inch horizontal shifts that we are talking about here.

However, they do have some small effects. Properly measuring those effects, though, I have found to be nearly impossible with the PITCHf/x data. It is very hard to distinguish pitch type effects, for example, from effects due to the catcher target and the ball-strike count. The ball-strike count can be eliminated or adjusted for, but without catcher target data, that effect is tougher to adjust for.
Apologies if this has been covered and I missed it, but have you looked at effects of batter positioning yet? Maybe LHBs typically stand closer to the plate.
I didn't cover it in this article, but yes, LHB do stand a little closer to the plate. I talked about that in more detail in my article on hit batters.

I had thought that maybe what you suggest was a factor in the shifted strike zone. My speculation was that because the left-handed batter stood closer to the plate, the umpire had to shift closer to the middle of the plate, shifting his strike zone in that direction along with it. But I didn't see that in operation in the positioning data for these three umpires. It's possible that (1) it does work that way, but this sample of data is too small to show it, or (2) the batter position relative to the plate affects the strike zone in some other manner than the umpire positioning.

I discussed those possibilities in my original strike zone (linked in the second paragraph of this article) and concluded that catcher target was the most likely explanation. For one thing, I observed a number of exceptions to the "stands close to plate, strike zone shifts outside" rule. But batters who saw a lot of outside pitches almost all had their strike zones shifted outside. In that article, I discussed some of the questions of cause and effect around that.
Former Umpire here ...
Couple things - just like a ball hit directly at an outfielder - it is difficult to judge/see pitches unless at an angle (Monocular vision) ...

That is why umpires get in the slot ...when you have a left handed hitter up (yes they do crowd the plate more generally) ..

When a hitter crowds the plate - and the catcher is a bigger guy ...the umpire has to compromise his angle at times to see as well as he can ...

Conversely, on balls on or near the "inside rail", the left handed hitter sometimes gets a little break because he is so close to the plate - and with the umpires' compromised view - it seems the inside pitch is exploding right at them (like a ball hit directly at an outfielder - they tirn sideways to guage depth) ....

Additonally, If the umpire is right eye dominant - then the outside pitch to a lefthanded hitter is not seen with the same confidence as an outside pitch to a right handed hitter.

Ironically, most left-handed hitters in upper level baseball have longer arms - coincidence perhaps.

Bottom line - as long as the umpire is consistent - the players adjust - its just Baseball, always has been Baseball

Its kinda cool though that a statistical analysis produced results that validated what we umpires were able to discern frequently over a few beers