The Strike Zone in 2015

The strike zone has been expanding in recent seasons, and many believe that it has resulted in less offense in Major League Baseball. Ian York has looked at how the strike zone is being called and how catchers can expand it further. In this piece, he observes the strike zone in 2015 to see if it is still expanding.

At the beginning of the 2015 baseball season, we reviewed the changes in the strike zone as umpires call it, and showed that the de facto strike zone has become significantly larger since the introduction of PITCHf/x in 2008. In fact, the bottom of the strike zone expanded abruptly between 2011 and 2012, and then again between 2013 and 2014. It is possible that the larger strike zone is an important factor in reducing offense over the same period, and we said at the time:

So one of the things worth watching this year is what happens to the strike zone. We probably need at least a month’s worth of data to get a clear answer, but because the size changes mainly happen in the off-season, the first month should be enough to tell.

We are just about a month into the 2015 season, so it might be possible to get an idea of whether and how the strike zone has changed since last year.

To do this we will use the same charts as last time, showing the difference in the probability of a strike call in the region around the plate, and compare that to the 2014 calls. For example, here is what 2013 looks like, when strike probability is compared to 2014:

Strike Zone Differences Update IMG 1

These plots are from the umpire’s point of view. The areas in red were more likely to have a strike called in them in 2013 than in 2014, while the blue areas were less likely to have a strike called in them. The grey outline shows the strike zone as it was called in 2014. The zone in 2014 was larger than in 2013, especially at the bottom, so in 2013 the bottom region was less likely to have a strike called in it.

Here are the same plots comparing 2015 through May 12th to 2014:

Strike Zone Differences Update IMG 2

In contrast to the 2013/2014 strike zones, the differences between 2014/2015 are quite minor. It appears that the top of the zone has been slightly less likely to have a strike called in it and that the bottom has been slightly more likely to have one called there, but the key word is “slightly”.

It seems that the strike zone in 2015 is going to be quite similar to that in 2014, which is probably a good thing. Batters have enough on their minds without worrying that the strike zone they have learned over their careers might still be gradually migrating on them.

Ian York has also looked into pitchers’ repertoires and the effect debut age has on performance.
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