For the past decade, Major League Baseball pitch data have been collected and made publicly available. During much of this era, the strike zone has grown and grown, primarily by seeing the bottom of the zone drop and drop.

With the first half of the 2017 regular season in the books, it feels like an appropriate time to check in on the way the strike zone has been called by Major League Baseball home plate umpires relative to recent seasons.

STRIKE ZONE HISTORY, 2009-2017 Year Strike Zone Size Strike Zone Size Below 21” K% BB% R/G 2009 435 0 18.0% 8.9% 4.61 2010 436 6 18.5% 8.5% 4.38 2011 448 11 18.6% 8.1% 4.28 2012 456 19 19.8% 8.0% 4.32 2013 459 30 19.9% 7.9% 4.17 2014 475 47 20.4% 7.6% 4.07 2015 478 50 20.4% 7.7% 4.25 2016 474 45 21.1% 8.2% 4.48 2017 464 43 21.6% 8.5% 4.66 SOURCE: Baseball Savant All measurements in square inches. 2017 data up to the All-Star Break, excluding all games played in Atlanta due to suspicious pitch location measurements.

The strike zone contracted slightly in 2016 and has gotten still smaller this season. It is now being called in a manner that makes the typical strike zone smaller than it has been since 2013. The overall size continues to decline, and the bottom of the zone, which has been the primary area of expansion during this era, is also slowly receding.

As the strike zone size has finally started to reverse its expanding ways, the rates of all Three True Outcomes categories have been climbing. With all else being equal, a higher walk rate tied to the smaller zone size makes sense, and we might expect scoring to trend upward as it has. Of course all things have not been equal, as subtle changes to the construction of the baseball have fueled the well-publicized home run bonanza during the same time frame as the zone has begun to contract. This has resulted in the first half of 2017 playing to the highest number of runs per game of any year since the strike zone started to expand. In addition, steadily improving pitch velocities and a growing acceptance of the potential advantages to trade off more swings and misses for more damaging batted balls when contact is made has meant league strikeout rate shows no sign of leveling off.

Here are images of the called strike zone to date this season from the umpire’s perspective:

There are a couple of observations to make about the size and shape of the strike zones when broken down by batter handedness. The first is that right-handed hitters are faced with a larger called strike zone than their left-handed hitting counterparts. This has always been the case — at least since pitch data has been available — but the gap has widened in the past few seasons.

Recent Statistics for (Right Handed Hitters – Left Handed Hitters) Year Strike Zone Size K% 2009 6 -0.4% 2010 5 -0.7% 2011 5 0.0% 2012 7 -0.2% 2013 5 0.1% 2014 17 1.4% 2015 23 1.3% 2016 10 1.2% 2017 14 1.7% SOURCE: Baseball Savant All measurements in square inches. 2017 data up to the All-Star Break, excluding all games played in Atlanta due to suspicious pitch location measurements.

After years of a small but consistent difference in size between the zones for right-handed batters and left-handed hitters through the 2013 season, for the fourth season running right-handed hitters have had to deal with an extra strike zone area into double digits of square inches. In accordance, what had been a neutral situation in strikeout rate between batter handedness through 2013 has become an environment where right-handed hitters strikeout at a notably higher rate than left-handed hitters as their relative zone sizes have further diverged.

The second observation from the strike zone images, for the keen reader who recalls what the left-handed hitter strike zone has looked like in years past, is that the area off the outside part of the plate, the so-called “lefty strike”, looks far less prominent. As a visual reminder, contrast the left-handed hitter strike zones from today with that from a decade ago, again from the umpire’s perspective:

The bottom of the zone is where the bulk of the expansion has been and justifiably where the most attention has been paid, but as of this season the asymmetry that has characterized the strike zone for left-handed batters during the pitch tracking era is nearly gone. In fact, the aggressive skimming off of the “lefty strike” area in the past few seasons coupled with a far less concerted effort to add on to the inside edge of the left-handed hitter strike zone is what has driven the widening of the size gap between right-handed batters and left-handed batters. The following graphic shows the percentage of pitches that have been called strikes for left-handed hitters, by month, in the areas marked in green in the image background:

The months of 2017 are marked in red for emphasis on this season’s data. Drawing your attention to the right side of the image first, you can see the consistent trend toward calling the outside edge of the left-handed hitter zone more in line with the rulebook strike zone. For reference, the outer edge of the plate would be aligned with the right side of the marked green area, at -0.75 feet. This pattern is continuing in force in 2017, with every subsequent month seeing fewer strikes called off the outside corner of the plate.

The left side of the image shows the haphazard manner in which umpires are attempting to call pitches on the inside corner of the plate as strikes. The span of consecutive regular season months starting with the last month of 2016 into the first two months of 2017 were the three months with the highest called strike percentage on this inner part of the plate in the past decade. However, it is worth noting that this momentum stalled in June and the first half of July, with inside edge called strike rates reverting to previous levels. The lack of sustained strike calling on the inside part of the plate to left-handed hitters appears to be the most obvious area remaining where the league could stand to improve in steering its strike zone toward the rulebook definition.

One final demonstration of the changes to the strike zone in 2017 compared to last season is the following heat map from the umpire’s perspective, where blue indicates more balls called this season and red indicates a higher rate of called strikes in 2017:

From this view, it is clear that the zone for right-handed hitters has also been shifting slightly toward the outside corner this season. Given that the shift has been in the same absolute direction for all batters, and the fact that Statcast took over from PITCHf/x for pitch tracking starting this season, it begs the question of whether there is a consistent measurement bias in the horizontal plane between the two systems. A study in April found some signs of Statcast measurement challenges that were said to have been corrected in the opening weeks of the season. One simple test for a systemic bias is to check the average horizontal position of all pitches as they cross home plate by season:

Average Horizontal Pitch Location, 2009-2017 Year Average Horizontal Pitch Location 2009 -0.091 2010 -0.061 2011 -0.063 2012 -0.083 2013 -0.066 2014 -0.062 2015 -0.036 2016 -0.027 2017 0.037 SOURCE: Baseball Savant 2017 data up to the All-Star Break, excluding all games played in Atlanta due to suspicious pitch location measurements.

The average pitch location as measured by Statcast in 2017 is about 3/4″ closer to the left-hand batter box than the average pitch location as measured in 2016. While there has been a trend since 2012 of reported positions trending in that direction, the largest previous single season shift was about 1/3″. I would take from this that it is likely there is at least a small systematic difference in the manner in which the horizontal pitch position at the front of home plate is being reported this season. With this discovery in mind, the extent to which the left-handed hitter strike zone has looked more symmetric about home plate in 2017 than 2016 may need to be taken with a grain of salt.

Another interesting perspective from which to analyze the strike zone involves looking at the “best” called games of the season. In this context, what can be measured is how closely each individual game is called relative to the league-wide consensus strike zone as measured in this article. Pitch calls are weighted by their difficulty, such that pitches called in areas where there is greater consensus are given a low positive weighting when they agree with the large majority, but a high negative weighting when they disagree. These are the five games which were called closest to the typical 2017 Major League Baseball strike zone:

Most Expected Called Games, 2017 Game Date Home Team Away Team Home Plate Umpire 5/3/2017 Padres Rockies Mark Carlson 5/24/2017 Phillies Rockies Tom Woodring 4/18/2017 Reds Orioles Jerry Layne 6/12/2017 Padres Reds Chad Whitson 6/24/2017 Marlins Cubs Phil Cuzzi SOURCE: Baseball Savant

Mark Carlson called just three pitches out of 129 chances opposite to what the typical call has been for those pitches this season when he worked behind the plate on May 3rd in San Diego.

An important realization that occurred as part of going through this game-by-game analysis, was that pitch positions at home plate as measured and reported from games at the new SunTrust Park in Atlanta in 2017 were notably inconsistent with the rest of the league data from this season. The majority of games that evaluated as most divergent from the consensus strike zone were in the Braves’ new stadium, and were spread throughout all months leading up to the All-Star Break. For this reason, the league-wide strike zone numbers presented in this article were recalculated with all games at SunTrust Park removed. For full disclosure, this led to both the overall strike zone size and its size below 21″ being 2 square inches smaller than when the games in Atlanta were included in the aggregation.