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The study used balls and strikes data from the past 13 seasons to determine the rate of missed calls against certain players. Snowdon determined which should-be strikes were called balls, which balls were erroneously called strikes and then looked at the race of the umpire, batter and pitcher. What Snowdon found was that umpires made more advantageous calls when their race was the same as the person receiving the advantage.

The difference amounts to 0.3 percent, which, while small, can be significant over the course of a single season, according to Robert Arthur of Baseball Prospectus.

These effects are small, but also large enough to be noticeable. Mistaken calls are about 0.3 percentage points more likely due to race effects, according to the study. Snowdon estimates that umpires called about 18,000 pitches differently over the 13-year period of the study because of racial bias, meaning a little more than a thousand changed calls per year. Any individual player might only receive a handful of these in a season, but for Black players in the league already struggling against discrimination in other regards, any additional barrier is a significant problem.

Arthur also pointed out that 90 percent of the umpires in MLB were white over the period in which the study was conducted. 

While white umpires gave more favorable calls to white players, that was also the case with Latino umps. Snowdon's study found that Latino umpires also show bias toward non-white players. 

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