Are Umpires Expanding the Strike Zone as the Season Goes On?

David Ortiz of the Boston Red Sox recently complained that the strike zone has been expanding as the season has gone on. He stated that the zone is growing in order to speed up the games. I decided to have a look to see if there was validity to this statement

I examined the area of the strike zone where about 50% of the time a pitch taken is called a strike and the other 50% of the time it is called a ball. This zone is just smaller than the rule book strike zone. Here is the percentage of called strikes strikes compared to the sum of called strikes and balls for each month and year:


Note: Combined September data should be taken with a grain of salt because none is available from 2010 and this year more pitches generally are being called strikes.

As a rule, the number of pitches called strikes in this zone increase by ~ 2.5% from the beginning of the season to the end. The called balls and strikes thrown into this zone account for only 12.4% of all pitches throw over the time frame. This works out to 0.3% of all pitches during a game — or, in a 300-pitch game, one extra pitch that is called a strike versus a ball. Not that much difference.

To further show the difference, here are the called strike zones of all umpires from 2008 to 2010 for right-handed batters in the months of April and August.

Note: The numbers indicate the decimal format of the percentage of pitches that are called strikes. The circle means nothing, it is just used for visual reference.





The zone extends a bit on the right and left parts of the plate, but not that much. There seems to be some increasing of the zone over the season, but it is not that much for teams or players to worry about.

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Jeff writes for FanGraphs, The Hardball Times and Royals Review, as well as his own website, Baseball Heat Maps with his brother Darrell. In tandem with Bill Petti, he won the 2013 SABR Analytics Research Award for Contemporary Analysis. Follow him on Twitter @jeffwzimmerman.

15 Responses to “Are Umpires Expanding the Strike Zone as the Season Goes On?”

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  1. bill says:

    2.5 percentage point increase is actually kind of noticeable, but look at April 2008 vs. August 2010. 6.8 percentage point difference, kind of amazing.

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    • Jeff Zimmerman says:

      It was a 2.5% change of pitches only in that zone, which accounts for a 0.3% of all pitches.

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  2. J-Doug says:

    Not sure why you’d only look at pitches inside the zone you determined rather than simply all called pitches, since it’s the total population of called pitches that determines the strike zone in the first place.

    In other words, if the umpires start making more mistakes on balls thrown well outside the zone, this is just as relevant to your hypothesis. Nevertheless, you exclude them.

    This is ultimately important because you base your evaluation of the impact of these errors on the ratio of called to all pitches, which you yourself deliberately shrunk.

    I would suspect that the frequency of errors on called pitches still doesn’t have much of an impact, but it would be more convincing if you looked at the whole population, and I don’t really see much of a reason for reducing it in the first place.

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    • Jeff Zimmerman says:

      I did look at pitches when doing the initial work. This zone was the only one that showed any kind of change across the season. Here is the numbers for all 3 seasons inside of the zone I looked at:

      April 70.1%
      May 71.3%
      June 70.8%
      July 71.1%
      August 70.9%
      September 70.0%

      If anything, I feel I was really stretching it to make the conclusions I did.

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  3. AJS says:

    How can you look at the part of the zone where 50% of pitches are called strikes, and have that part of the zone be where 52.5% of pitches are called strikes? It’s self-contradictory.

    It would have been better to look at the area of the zone (in square inches or whatever) where 50% of pitches are called strikes in April vs. the area of the zone where 50% of pitches are called strikes in August.

    Incidentally, since there does seem to be a trend, I wonder what’s behind it. Are pitchers sharper this year, so umpires are more likely to give them borderline calls? Is it to balance out the effect of hot weather that generally gives hitters an advantage in July/August? Any other possibilities?

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    • Jeff Zimmerman says:

      The method of looking at the areas of the 50% zone would be nice, but it is beyond my programming/querying ability. I had to stay with the same zone for each set of data.

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  4. Watcher says:

    Not sure if something like this has been done before (not even sure if the evidence is available) but I would be interested in knowing if specific teams get smaller/bigger zones.

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  5. Cidron says:

    Dunno Watcher…
    Why not ask Glavine, Smoltz, and Maddux during their Atlanta days.. (or better yet, ask their post season foes). There were some m i g h t y wide strikezones.. No, not current, but, to your question, I am sure that there are some that do get…. more favorable (or, less favorable) zones. Some umps have biases towards certain teams.. I am sure Ozzie Guillen can provide a few names of umps :p

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  6. CircleChange11 says:

    You mean like how if Wade Boggs didn’t swing, it wasn’t a strike? *grin*

    I think most umpires legitimately try to maintain a consistent strikezone, and probably do a very good job of it. Umpires would have to be almost super-human to maintain a perfectly consistent strikezone throughout the season, and my guess would be that it is much easier to allow a strikezone to “expand” rather than “contract” as the year goes on.

    How hard can it be to have sensors in the jersey (tops of knees and letters) and on the corners of the plate, with a chip in the ball, and a red/green signal signifying balls and strikes? Yes, I’m laughing to myself. MLB may have that by 2347, 300 years after the other sports would have been using the technology for boundary and zone calls.

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  7. pft says:

    Glad someone finally looked at this, but…

    “There seems to be some increasing of the zone over the season, but it is not that much for teams or players to worry about.”

    I disagree, since you have not factored in that an expanded strike causes hitters to chase balls out of the strike zone. There is a multiplier effect that dwarfs the magnitude of the number of called strikes out of the zone. And while you can say players should not worry about it, Papis comments indicate they are.

    Players adjust to an umpires strike zone. The strike zone is what they say it is. A couple of balls called strikes early in the game, and players are swinging at balls out of the zone with 2 strikes.

    It is a plausible explanation for a drop off in offense in the AL. I would recommend looking at the pitch data for the AL only since the NL seems to be little changed offensively and might be dampening the numbers for the AL

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    • But it isn’t “a couple of balls called strikes early in the game.” The numbers say it’s about ONE pitch difference in the enitre game. No way they notice and adjust to that.

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  8. Steve says:

    I could see why Ortiz would think this, seeing how no one on the Red Sox has EVER been punched out on a legitimate called strike 3.

    I am basing this on their reactions, which I trust to be accurate.

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  9. jalbouty says:

    I think I’d be interested in a study of strike zone as a function of game time temperature. I’ve heard a lot of people make the assumption that “the ump wants to get out of here” when borderline strikes are called during hot games.

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  10. Awz says:

    This is a very interesting study. Do you think every season has a little bit of flux in the zone?

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