Molyneux: Umpires aren’t compassionate; they’re Bayesian

From Guy Molyneux at Baseball Prospectus on February 24, 2016:

Baseball fans have long known, or at least suspected, that umpires call balls and strikes differently as the count changes. At 0-2, it seems that almost any taken pitch that is not right down the middle will be called a ball, while at 3-0 it feels like pitchers invariably get the benefit of the doubt. One of the earliest discoveries made possible by PITCHf/x data was the validation of this perception: Researchers confirmed that the effective size of the strike zone at 0-2 is only about two-thirds as large as in a 3-0 count.

One common explanation offered for this pattern is that umpires don’t want to decide the outcome of a plate appearance. Preferring to let the players play, this argument goes, umpires will only call “strike three” or “ball four” if there is no ambiguity about the call. As Etan Green observed at Five Thirty Eight: “Umpires call balls and strikes as if they don’t want to be noticed.” The data, however, do not support this theory. The called zone shrinks in all pitchers’ counts, even those with only one strike. Similarly, the impact of ball three on zone size is no greater than that of ball one or two. So umpires do not have any particular aversion to ringing up a batter, nor to granting him a free pass. Something else is going on.

A theory that fits the data better, first offered by John Walsh in 2010, is that umpires are putting their thumb on the scale for either the pitcher or the hitter, depending upon the situation. Suggesting that “major league umpires are a compassionate bunch of guys [who] can’t help pulling for the underdog,” Walsh argued that umpires act unconsciously to help whomever is at a disadvantage at any given count, giving pitchers a more generous zone in hitters’ counts and vice versa.

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Originally published: February 24, 2016. Last Updated: February 24, 2016.