Roegele: The “living” strike zone in baseball

From Jon Roegele at Baseball Prospectus on July 24, 2013:

The strike zone is always a hot topic within the baseball community, and the attention on called strikes has only intensified since the public introduction of the PITCHf/x tracking system in 2007.

Thanks to this technology, the position at which every pitch crosses the plane of home plate is reported by means of a fitting algorithm and extrapolation. While the reported measurement is not perfect, due to potential error from a number of sources (including fitting error and camera characterization error), the reported data gives us a very good idea of pitch locations and whether pitches were deemed balls or strikes.

Numerous studies on the strike zone have been published over the past several years. Some of these studies have shown that the strike zone is called differently based on factors such as batter handedness and the count. This phenomenon fascinates me, so I decided to examine the called strike zone across a range of factors to investigate which ones appear to consistently affect its size and/or shape.

To assess the strike zone at the most granular level, I divided the plane at the front of home plate into 1×1 inch squares. I then binned every called pitch into its appropriate square by its location and calculated the percentage of pitches in each grid location that were called strikes. If more than half of the pitches in a cell were called strikes, then that cell was included in the strike zone. With this framework in place, I then adjusted one variable at a time through a range of values to examine its potential effect on the size and shape of the called strike zone. With one-square-inch grid locations, it is simple to sum the cells and report the called strike zone in terms of its area in square inches.

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Originally published: July 24, 2013. Last Updated: July 24, 2013.