Are Baseball Umpires Biased?

Just how good are professional baseball umpires? They're actually remarkably good, according to a study by Tobias Moskowitz and Jon Wertheim, who in their book, "Scorecasting: The Hidden Influences Behind How Sports Are Played and Games are Won," compared how pitches were called with how they should have been called.

How were they able to conduct such a study? Some of you may know that in 2007 Major League Baseball installed cameras in ballparks to track the location of every pitch. These cameras are accurate to a centimeter, which allows fans watching on TV to become indignant when calls go against their team, and researchers, such as Moskowitz and Wertheim to study just how good umpires are.

What they found is that umpires call pitches correctly 85.6% of the time, which is pretty amazing considering that the average pitch leaves the pitcher's hand at about 92 mph and crosses home play at about 85 mph.

It also turns out, however, that umpires are a little biased. Umpires are much more likely to make mistakes when there are three balls on the batter than when there are two or fewer. Specifically, when pitches are outside of the strike zone, the normal error rate is 12.2%, but when there are three balls on the batter (not including those times when there is a full count), umpires "erroneously call strikes on the same pitches 20 percent of the time" ("Scorecasting," p. 15).

A similar phenomenon occurs when batters have two strikes on them, except in these situations umpires are more likely to strikes balls than when batters have less than two strikes on them. Specifically, when batters have two strikes on them (not including those times when they have a full count), umpires make an incorrect call 39% of the time, which is more than double their normal rate.

In other words, when there are three balls on a batter, the umpire's strike zone expands, and when there are two strikes on a batter, the umpire's strike zone shrinks.

Why? Moskowitzh and Wertheim believe that it is because umpires prefer not to insert themselves in the game. That is, they believe that umpires want batters to place the ball in play, so they are less likely to call a fourth ball or a third strike.

Baseball is not the only sport or issue that Moskowitzh and Wertheim consider in their book. They examine other topics, such as why teams from most sports win more at home than on the road (highest % = soccer = 69.1%; lowest % = baseball = 53.6%), the increased probability that adding a superstar will lead a team to make the playoffs, get to the finals and winning a championship (it isn't as high as you probably think -- can anyone say, Miami Heat?) and why the Chicago Cubs appear to be cursed.