The Evolving Strike Zone in the PITCHf/x Era

Each baseball season brings with it stories to follow, from promising rookies, veterans in search of a milestone or career resurrection, or the unexpected effects of minor changes. One story to watch is the evolving strike zone and its effects on offense, pitching, and defense.

The baseball rulebook defines an official strike zone, but umpires do not necessarily call balls and strikes by the rulebook. From 2010 to 2014, the strike zone, as called by umpires, grew larger, while offense in baseball fell by about 10%. Specifically, the de facto right-handed batter’s zone in 2014 was a little over 11% larger than it was in 2008, while the left-handed batter’s strike zone was between 8% and 9% larger. This was estimated using PITCHf/x data for called strikes and balls, identifying the region where the probability of a strike was 50%, calling this region the edge of the strike zone, and calculating the area inside that region. 

Here is how this looks. This is from PITCHf/x called strike and ball data. I calculated the probability of a pitch being called a strike in a matrix across the area, for both 2014 and 2008, and then subtracted the 2008 array from the 2014 array:


These plots are from the umpire’s point of view. The areas in red are more likely to have a strike called in them in 2014 than in 2008, while the blue areas are less likely to have a strike called in them. The grey shape shows the strike zone as it was called in 2008. The 2014 zone to both left and right-handed hitters has dramatically expanded at the bottom and compressed slightly on the edges, especially on the outer part of the plate.

However, this change did not happen all at once. Here is how the zones have changed since 2008. The 2D histogram calculates strike probability for each point in a matrix by looking at consecutive two-month slices of each season, using PITCHf/x data for the appropriate months. Blue indicates a low probability of a strike call, green indicates a high probability, and red shows the area where there’s approximately a 50-50 chance of a pitch being called a strike – that is, on the edges of the actual strike zone. The average strike zone for each year is shown in white. Because the granularity was a little lower for the two-month slices than for a full year, I also applied a smoothing algorithm to remove some of the small sample size spikes and notches:

kzone_by_thirds fixedWhile the strike zones grew some 10% larger – by what is surely a complete coincidence – the strikeout rate per nine innings has also increased by a very similar amount amount over that period: a 13.2% increase from 6.8 to 7.7 per Baseball Reference. With the increased strike zone and strike out rate, overall offense has dropped significantly as well. Runs per game have decreased from 4.65 in 2008 to 4.07 in 2014.

A lot of the horizontal compression happened between 2010 and 2011 with the zone actually shrinking slightly in that period. But between 2011 and 2012 there was a big expansion in the bottom of the zone. Here are the differences in the probability of a strike call in that one-year period (showing the 2011 zone as the grey lines):


It’s not as spectacular as the overall jump from 2008-2014, but there is still a significant change for a single year.

Then, between 2013 and 2014, there was another increase at the bottom of the zone, especially for right-handed batters, who generally have a bigger zone than lefties:


It is also worth noting that the zones have changed slightly during the season as well, though not nearly as much as in between seasons.

The umpires and the MLB administration are likely the only people who know exactly why the zone has changed like this, and they are not talking. However, there are two critical points: First, the zone in 2014 is much closer to the official zone. Prior to 2009, the bottom of the zone was not being called according to the rulebook guidelines:

The STRIKE ZONE is that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants, and the lower level is a line at the hollow beneath the kneecap. The Strike Zone shall be determined from the batter’s stance as the batter is prepared to swing at a pitched ball.
Rule 2.00: The Strike Zone

Second, this has happened in a period when umpires are being evaluated on their ball and strike calls as compared (by PITCHf/x) to the official (rulebook) zone. It’s reasonable to say this is something the umpires are doing deliberately, and that it is a reaction to being evaluated by PITCHf/x data.

That brings us to the question: Is the bigger strike zone responsible for the drop in offense across the league? Probably not entirely – K/9 started creeping up in 2009, before zone expansion became a major factor – but it is likely a major contributor. Jon Roegele – one of the first to notice the expanding strike zone – at Hardball Times, and Brian Mills, of the University of Florida, both make strong cases that the drop in offense is significantly, but not completely, driven by the expanding strike zone.

There is another question worth exploring. Is the new zone especially hard on rookies attempting to adjust to Major League play? PITCHf/x is installed in some minor league parks, but not all of them. That should mean that umpires in the minors do not face the same incentive to expand the minor-league strike zone. Unfortunately, unlike the majors, minor-league PITCHf/x data are not available to the public, and without PITCHf/x, we cannot objectively verify that idea yet. But supporting that thought is the observation that AAA ball has not seen the same drop in offense as has MLB.

The implication is that when rookies come up from Triple A to the majors, not only do they have to contend with better pitchers, a faster game, and more intense scouting and planning, but they also have to deal with a brand-new strike zone that is dramatically bigger than any zone they have seen before. In 2014, several highly-rated rookies struggled offensively in the majors. Again, it is not likely that the expanded strike zone is solely responsible, but it was probably a contributing factor to those struggles.

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. Is this going to be like 2009-2010, when the zone either stayed constant or even shrank a tiny bit? Is it going to be more like 2012 and 2014, when the zone was bigger as soon as the season started? Or – who knows? – maybe the strike zone will start to shrink a little, now that people are looking at it and complaining. The answer to that may turn out to have the biggest impact on baseball offense in the coming season.

Follow Ian on Twitter @iayork

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Ian is an immunologist and virologist who lives in Atlanta with his wife and two sons. Most of his time is spent driving his kids to baseball and soccer games, during which he indoctrinates his children on the glories of Pedro Martinez, the many virtues of the Montreal Expos, and other important information.