A few months ago, I posted about a study showing implicit racial bias in NBA referees’ calls. Now it’s baseball’s turn, because yesterday reports of study by Parsons et al.1 that shows analogous results for home plate umpires began popping up all over the media.

The study is pretty straightforward, though the data analysis must have taken forever. I’ll let Parson’s et al. tell you what they did:

There are 30 teams in Major League Baseball, with each team playing 162 games in each annual season. During a typical game each team’s pitchers throw on average roughly 150 pitches, so that approximately 730,000 pitches are thrown each season. We collected pitch-by-pitch data from ESPN.com for every MLB game in the three years 2004-2006.2 For each pitch we identify the pitcher, pitcher’s team, batter, batter’s team, pitch count, score, inning, and pitch outcome. We classify each pitch into one of seven mutually exclusive categories: Called strike, called ball, swinging strike, foul, hit into play, intentional ball or hit by pitch. We supplement each pitch observation with game-level information from ESPN.com box scores including the stadium name, home team, away team, team standings, and the identities and positions of all four umpires. In addition, for each pitcher’s appearance in each game we collect the exact number of innings pitched and the number of allowed hits, walks, strikeouts, homeruns, runs and earned runs. Finally, for each starting pitcher in each game we collect the game score, a composite index designed to summarize a starting pitcher’s performance. (p. 4)

Those three seasons yielded a total of 2,120,166 pitches. I feel sorry for the poor undergrads who had to sit and watch all of those! Forty-seven percent of the pitches were excluded because the batter swang or was hit by the pitch, or the pitch was intentionally thrown for a ball, leaving 53% of 2.1 million pitches to analyze. In addition to looking at whether each pitch was a ball or a strike, they also coded the race of the home plate umpire, batter, and pitcher. Using a regression model (see the basketball post for a brief description of regression) that takes only these into account, they find that when the umpire and pitcher are of the same race, the umpire is about .34% more likely to call a strike. That’s only a little more than half a pitch a game, but over all of those pitches, and when you take consider that it’s only one source of potential discrimination, it’s a potentially important effect.

Interestingly, the effect of same-race umpires goes away when the umpire’s calls are evaluated by QuesTec’s computerized system (only 35% of MLB ballparks had the system between 2004 and 2006), when crowds were larger, and when there were either two strikes, three balls, or both (meaning that the call could potentially end the bat). Parsons et al. argue that these results suggest that the cost of a decision will affect the influence of implicit racial bias. Specifically, the data implies that costlier decisions will display less bias, a finding that could have a wide range of implications in research on implicit biases.

¹Parsons, C.A., Sulaeman, J., Yates, M.C., &Hamermesh, D.S. (Unpublished Manuscript). Strike three: Umpires’ demand for discrimination.