Choiniere: The swing zone is connected to the strike zone

From John Coiniere at Baseball Prospectus on May 11, 2016:

By now, it’s been well established that the strike zone (as called, of course, not as defined) has expanded dramatically in the past few years. It’s further been established that this increase has principally come from more low pitches being called strikes. It’s also been suggested that this expanded zone has been at least in part responsible for the drop in overall scoring seen in the same time frame, though the uptick in runs per game at the end of last year (which has continued this year) at least partially contradicts this. It seems obviously worthwhile, then, to try to examine what sort of effect the new zone is having on the behavior of batters—and especially their swing rates.

To do this, I want to create something analogous to the strike zone that instead looks at swing behavior. By looking at whether the “swing zone” has expanded as well, I might be able to see if and how batters are reacting to the new strike zone reality.

To look at this “swing zone”, I’ll be using a method that’s similar but not identical to the way Jon Roegele has defined strike zone area in his work looking at strike zone expansion in the PITCHf/x era. Jon divided the plane of the plate into one-inch squares and used a strict threshold of taken pitches being called strikes in that box to determine whether it should be included as part of the zone. I use the same grid system, but rather than a threshold, I’ll be using the frequency of swings on all pitches in each box and summing them over all boxes to establish the total size of the zone in square inches. I’ll also use this general method to establish strike zone size when necessary. All data shown are from combined regular season and playoff games except if otherwise specified.

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Originally published: May 11, 2016. Last Updated: May 11, 2016.