Starting in 2010, the MLB strike zone has changed size and shape dramatically, probably because umpires started using (and being evaluated on) electronic review of their strike calls. There were large changes in the zone between 2010 and 2011, and from 2013 to 2014. However, there was very little change in the size or shape of the strike zone from 2014 to 2015. Have the umpires finally settled on a zone they’re comfortable with?
To evaluate the strike zone, I use PITCHf/x data to collect the location of all the regular-season called strikes and balls, and calculate the probability of a strike being called in each sub-region around the strike zone. The area in which the probability of a strike being called is around 50-50 marks the edge of the de facto strike zone. This is what it looked like in 2016 for left-handed and right-handed batters. Areas in blue have a very low probability of being called a strike; those in green were very likely to be called strikes; and the region in which balls and strikes are equally likely is red. These charts are from the umpire’s viewpoint, so the batters would be standing in the middle of the two charts:
That’s not very informative on its own. Here we can compare 2008 (before the zone started to expand) to 2015 and 2016:
The 2015 and 2016 zones are not identical, but there isn’t a lot of difference between them; both are much taller and narrower than the 2008 zone was.
We can also directly compare the zones and highlight differences. Here, regions in which a strike was more likely to be called in 2016 than in 2008 are shown in red, and regions in which strikes were less likely to be called are blue. The intensity of the color represents the probability:
Again, this highlights the taller, narrower zone that was called in 2016 compared to 2008. The zone expanded most at the bottom while the sides shrank equally dramatically, especially for left-handed batters; the top of the zone also moved upward, but didn’t change as much as the bottom did.
Now doing the same comparison for 2016 to 2015:
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.