With the 2015 season in the books, it’s a good time to reflect on what changes if any occurred during the season. The strike zone is something that has been evolving since PITCHf/x was installed in all MLB parks. Ian York takes a look at the data to determine the changes in the 2015 MLB strike zone.
We have looked at the changing strike zone several times this season. We showed that the strike zone has changed, with significant size increases between 2010 and 2011, and from 2013 to 2014. We showed circumstantial evidence that the changes reflected umpires’ new feedback from PITCHf/x data. And during the 2015 season, we looked for early indications of more changes in the zone, and their effects on the game and players. With the regular season closed out, and with some umpires’ questionable zone interpretation in the playoffs, we can look at the final strike zone as it was called in 2015.
In this gif, we look at all the called balls and strikes from the 2014 and 2015 regular seasons. Areas where a pitch was almost certain to be called a ball are shown in blue; areas where the calls were almost always strikes are green. Areas where the chances of a pitch being called a strike are between about 40 to 60% – that is, the edges of the de facto strike zone, as umpires called it in each year – are shown in red.
As you can see, there is very little difference between the 2014 and 2015 zones. The 2015 zone seems to be a little bit more square: Umpires are identifying the corners of the zone, rather than letting it be rounded off. Also, they are treating the left and right margins of the zones as straighter lines than in 2014. But in terms of overall size, the zone has changed very little from the previous year.
(Since these charts average out many tens of thousands of called strikes, the occasional umpires’ mistakes should be lost in the noise. However, systemic effects – like the changing size of strike zones for different counts – are common enough to affect the overall picture, and help explain why the red outline is relatively wide.)
Changes are easier to see if we highlight differences. Here, regions around the strike zone that are more likely to be called strikes in 2015 are red, and those that are less likely to be called balls are blue. The intensity of the color reflects the amount of difference:
Pitches in and around the bottom of the strike zone are slightly more likely to be called strikes in 2015 than in 2014, while the edges of the zone have, if anything, shrunk slightly. But the differences are very small. Compare to the overall differences, looking at two-month intervals since 2008:
Both 2014 and 2015 strikes zones are much larger than in 2008, with the increase beginning in 2010, while there is little change in overall size since the beginning of 2014.
The size and shape of the 2015 zone differs for left- and right-handed batters, with righties having a significantly larger strike zone than lefties; this has been true for several years.
Strike zones are among the most controversial aspects of baseball. With PITCHf/x feedback available, umpires are now calling a strike zone that is much closer to the official zone than the wide but shallow zone they called before 2010. This larger zone is probably one (though not the only) cause of the reduced offense over the past few years. While it is possible that strike zones could, technically, be automated in the next few years, it is worth keeping in mind that this would undoubtedly lead to equally large and unexpected consequences.