status bias in baseball umpiring

Jerry Kim and I have an op-ed in Sunday’s New York Times about our new paper on status bias in baseball umpiring. We analyzed over 700,000 non-swinging pitches from the 2008-09 season and found that umpires made numerous types of mistakes in calling strikes-balls. Most notably, we expected that umpires would be influenced by the status and reputation of the pitcher, and this is indeed what we found:

One of the sources of bias we identified was that umpires tended to favor All-Star pitchers. An umpire was about 16 percent more likely to erroneously call a pitch outside the zone a strike for a five-time All-Star than for a pitcher who had never appeared in an All-Star Game. An umpire was about 9 percent less likely to mistakenly call a real strike a ball for a five-time All-Star. The strike zone did actually seem to get bigger for All-Star pitchers and it tended to shrink for non-All-Stars.

An umpire’s bias toward All-Star pitchers was even stronger when the pitcher had a reputation for precise control, as measured by the career percentage of batters walked. We found that pitchers with a track record of not walking batters — like Greg Maddux — were much more likely to benefit from their All-Star status than similarly decorated but “wilder” pitchers like Randy Johnson.

Baseball insiders have long suspected what our research confirms: that umpires tend to make errors in ways that favor players who have established themselves at the top of the game’s status hierarchy. But our findings are also suggestive of the way that people in any sort of evaluative role — not just umpires — are unconsciously biased by simple “status characteristics.” Even constant monitoring and incentives can fail to train such biases out of us.

You can can download the paper, which is forthcoming in Management Science, if you’re interested in learning more about the analyses and their implications for theories about status characteristics and the Matthew Effect.