Baseball’s Changing Strike Zone

From 1988 to 1995, the strike zone in Major League Baseball extended from the top of the batter’s knees, up to the midpoint between the top of his shoulders and the top of his pants. But in 1996, citing concerns that baseball games were getting too long, the league lowered the bottom limit of the strike zone to the hollow beneath the batter’s kneecap. They hoped it would result in more swings and quicker outs.

At the time, many players and analysts doubted that the new definition would be enforced. Cubs first baseman Mark Grace questioned the significance of the change:

“You can’t tell me that all those guys who have been calling a certain pitch a strike will suddenly be seeing it differently,” he said. “It’s not going to happen.”

Sportswriter Jerome Holtzman agreed, saying that “the umpires are going to keep calling them as they have in the past. A directive from league headquarters won’t be sufficient to effect a change.”

PITCHf/x and Zone Evaluation

The widespread adoption of Sportsvision’s camera-based PITCHf/x system in the late 2000’s seemed to prove the doubters right. The system enabled accurate, real-time tracking of pitch speed and location, and graphics that compared pitch locations to virtual representations of the strike zone became ubiquitous on TV and on the internet. The PITCHf/x system required an operator to manually set the top and bottom of the strike zone. As the operators set the bottom of the zone by the rule-book, it became clear that umpires were not calling that same strike zone. Umpires faced even more scrutiny starting in 2009, when Major League Baseball implemented Zone Evaluation, a system that graded umpires on how closely their calls aligned with the PITCHf/x strike zone.

The figures below use PITCHf/x data to contrast the strike zones called in 2009, when Zone Evaluation was just beginning, and the strike zone called during the 2017 regular season. The PITCHf/x zone differed noticeably from the called strike zone: The low strike was not called consistently, and the zone for right-handed batters was very different from the zone for left-handers. Right-handed batters faced a strike zone that crept in on the hands, whereas lefties saw more strikes being called off the outside edge of the plate. As umpires conformed to the PITCHf/x strike zone (shown in red), the called strike zone not only expanded downward to capture the low strike, it also shrunk horizontally as this asymmetry was corrected. In 2017, much fewer strikes were called inside to righties or outside to lefties, making the strike zones they face mostly mirror images of each other.