Roegele: The strike zone expansion is out of control

From SABR member Jon Roegele at The Hardball Times on October 13, 2014:

The playoffs are here, which means games are scrutinized more than at any other time in the season, with millions of extra eyeballs watching and every game crucial to the outcome of such short series. One of the most controversial aspects of each game is without a doubt the called strike zone, with seemingly no game passing without at least one team, if not both, unhappy with the performance of the home plate umpire.

We know by now that the strike zone has been expanding during the PITCHf/x era. I discovered this for the first time in the middle of last season, then followed up with a much more detailed examination of the topic at the end of 2013. Brian Mills was concurrently, independently discovering the same phenomenon, which he published in a wonderful academic paper earlier this year. All of these results concluded with data from the end of the 2013 season. Jeff Sullivan recently found evidence that the strike zone was continuing to grow in 2014, and I wanted to return to the same approach that I had used earlier to analyze the most recent regular season.

My approach involves splitting up the front plane of home plate into a 1” by 1” grid, and calculating the percentage of called pitches that crossed the plate in each grid cell that were deemed strikes. Every cell location that is called a strike more often than a ball is included in the strike zone for a given season.

What I found was perhaps not surprising – the strike zone did continue to expand, and that expansion was almost entirely due to the bottom of the zone dropping once again. What surprised me was that this season saw the largest single year increase in the size of the strike zone in the PITCHf/x era.

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Originally published: October 13, 2014. Last Updated: October 13, 2014.