So I play a lot of baseball. I also umpire a lot of baseball. As an umpire it is easy to make mistakes. Calling balls and strikes is difficult as you have to judge whether the ball is in the strike zone or not. This decision is made in an instant and you are judging a moving ball in the air. There are no actual lines you can look at to see the strike zone. Instead you are judging the ball in the air in relation to the home plate which is by the batters feet.  I am curious accurately major league umpires call balls and strikes and how does it affect the game.

As a player it is very annoying when an umpire makes a bad call against you. It can change your at bat drastically if a strike is called when the ball was miles outside and in the other batters box. Ok that was a bit of an exaggeration but you get the point! If the umpire is inconsistent its harder to leave a pitch – so then you are more likely to strike out swinging – especially if you end up swinging at bad pitches. On the other hand, when you are the pitcher its super frustrating if the umpire isn’t giving you the corners of the zone. You end up either walking a lot of batters, or you get crushed as you are throwing into the batters wheel-house.

The size of the strike zone will vary between umpires, but the most important requirement for a good game is that the umpire calls the zone consistently.

The MLB has used the PITCHf/x system for the last few years which is used by the TV broadcasters to where the ball crossed the plate. This system means there is now some data on how accurate umpires are calling the zone. According to an article on Beyond the Box Score from 2008-2013 umpires called “Ball” on a pitch that should have been a strike 13.2% of the time, and called “Strike” on a pitch that was outside the zone 15% of the time. This means that umpires have an average rate of 14% of the pitches they call wrong.

However, this statistic assumed that all umpires make about 3 bad calls per inning – which seems too high. The reason the number is high is its based on comparing the calls to the official zone as defined by the MLB. Whereas in reality umpires strike zones will vary in size. The umpires say they believe they get closer to 95% of calls correct.

The other interesting thing that the PITCHf/x data shows is that the umpires seem to adjust the zone according to the count. This means that when the count is 0-2 they are more likely to call a “ball” rather than a strike, accordingly the opposite applies where a strike call is more common when the count is 3-0. This article examines that data in more detail.