Following the introduction of PITCHf/x – and especially following the umpire’s union allowing video review of their ball/strike calls – the MLB strike zone rapidly began to change its size and shape. Between 2010 and 2014, the strike zone became much taller, and somewhat narrower; this was probably one cause of the reduced offense in that period.
After 2014, the zone has changed slightly, but there haven’t been the dramatic shifts witnessed in the few years before that. In fact, from 2015 to 2016, the zone actually shrank slightly, for the first time in the PITCHf/x era:
We can see that there were some differences between 2016 and 2015. For the first time since PITCHf/x data became widely available in 2008, the strike zone shrank slightly in 2016. Most of the change was at the bottom of the zone, which moved up for both left- and right-handed batters, while the top of the zone also moved upward slightly. The outside parts of each strike zone also shrank a little, while the inside either stayed unchanged or perhaps expanded a tiny amount. Overall, the zone shifted very slightly upward and inward in 2016, and became a little bit smaller, compared to 2015.
In the first half of 2017, the zone has again remained fairly constant. The chart below shows what umpires have called so far. In the blue areas, they almost always called a ball; in the green, almost always a strike. The red areas show the region where a pitch was roughly equally likely to be called a ball or a strike. (This is from the umpire’s viewpoint.) I’ve drawn a polygon (white) just about in the middle of the 2016 red zone, which I call the “strike zone” for 2016, and kept that constant while showing the called pitches from 2008 (before the zone began to enlarge), 2016, and the first half of 2017:
The difference from 2008 to 2016 is very obvious; not so much the difference from 2016 to 2017.
To highlight differences, here is a comparison plot (again from the umpire’s view), with differences between the two seasons shown. Regions in which a 2017 pitch was less likely to be called a strike than in 2016 are blue; areas in which a 2017 pitch was more likely to be called a strike are in red.
Basically, it looks as if the entire strike zone has shifted slightly over to the umpire’s right, without changing size significantly. The difference is quite small – less than an inch – and it may not be real. In 2017, a new pitch tracking system was put into place; instead of PITCHf/x, data are now collected by the TrackMan system. Perhaps on average, the new system is calibrated very slightly differently from the old one?
In any case, the important point is that the strike zone hasn’t changed its size significantly since last year, so batters and pitchers haven’t had to recalibrate their expectations, which is much more important than the apparatus.