As the 2016 regular season comes to a close, let’s look back at one of the most hotly debated and central aspects of the game: the strike zone. It was yet another eventful year for the strike zone, with news breaking part way through the season that MLB’s competition committee had voted to raise the lower boundary of the strike zone from the hollow below the knee to the top of the knee. The change is not guaranteed to be implemented next season, or even implemented at all, as all rule changes must also be approved by the league’s playing rules committee before being enacted.
The proposed change is at least in part a response to a strike zone that has been growing over the past several years, most notably seeing its bottom drop by a full width of a baseball since the start of the decade.
Here are the final numbers for the 2016 regular season compared with recent history:
|Year||Strike Zone Size (sq. in)||Strike Zone Size Below 21” (sq. in)||K%||BB%||R/G|
After six consecutive seasons in which the strike zone size increased, both in general and in particular at the bottom of the zone, the zone tightened ver so slightly this year.
Here are images showing the strike zone from the umpire’s perspective for left-handed and right-handed batters from this past season:
One observation from the table is that the 2016 strike zone ended up almost identical to the 2014 zone in overall size in addition to its size below 21 inches off the ground, an area I have specifically tracked over the PITCHf/x era. I thought it would be interesting to compare the 2014 and 2016 zones to see how close they look if we zoom in beyond these high-level similarities.
Clearly the outside “lefty strike” off the outside of home plate is still an area of contraction; it appears that for left-handed hitters the zone has squeezed in at the outer edge and been compensated with additional high strikes. There has also been a trend toward calling more strikes on the inner part of the plate for left-handed batters, as I discuss in more detail below. The right-handed zone looks to have been slightly tightened over both edges of the plate, with the largest concentrated area of extra called strikes also appearing at the top of the zone. So, while the overall size of the zones in 2014 and 2016 may be equal, the strike zone is still very much a living, breathing animal.
These latest data show some other visible breaks to recent patterns as well. After strikeout and walk rates tracked along with strike zone changes over the seven seasons prior to 2016, the most recent season witnessed another hike in strikeouts and a significant reversal of the plummeting walk total, despite the scarcely smaller zone. Runs per team per game also continue to rise, driven by the widely researched and discussed home run boom that began after the All-Star break in 2015. Since the potential contribution of the strike zone to the home run bonanza has already been ably investigated by Brian Mills on this very site, I will move on to other strike zone topics.
Batting Left is Right
Another aspect of the strike zone I have been tracking for quite some time is the growing disparity between the called zones for right-handed hitters versus left-handed hitters. After at least five seasons with a moderate yet consistent larger zone for right-handed hitters of about six square inches, the last three years have told a different story. The difference in strike zone size between batters with opposing handedness has expanded to three or four times what it had been previously, before settling at a 10-square-inch gap in the 2016 season.
|Year||Strike Zone Size (sq. in)||K%|
Technically speaking, the majority of this additional gap in zone size between righties and lefties has been caused by the contraction of the “lefty strike,” a region two to three inches off the outside of home plate for left-handed hitters that had been included in the strike zone for many years. While this is a welcome development, the same two or threeinches on the inner edge of home plate that has traditionally not been a called strike has not been added on emphatically enough to balance out the left-handed hitter zone.
That said, I have been tracking these numbers periodically throughout the season, and there was a clear effort over the last half of 2016 to call more strikes on pitches on the inner part of the plate for left-handed hitters. The following graph shows the percentage of called pitches deemed strikes in the grayed out area on the inner part of the plate to left-handed hitters by month throughout the PITCHf/x era.
July 2016 set the record for highest monthly called strike percentage in the grayed out area since PITCHf/x was implemented in MLB. This record was broken again in August of this season, before being blown away in September. So while the lack of calls to this part of the zone has led to a smaller relative zone size for left-handed batters as pitches off the outside corner began being correctly called as balls, there is now a trend post-All-Star Game in 2016 toward correcting that disparity.
As a result of this late season trend, the year-to-date gap in overall strike zone size between right-handed and left-handed hitters that was 19 square inches at the All-Star break shrunk to 10 square inches by season’s end.
With regard to this zone disparity, precisely in step with the jump in relative strike zone size for right-handed hitters in 2014, the strikeout rate difference spiked. After years of relative balance, right-handed batters are now striking out notably more often than left-handed batters.
A theory that I have held is that umpires appear to (subconsciously or not) make decisions that tend to keep the focus off themselves and balance the odds. We know that when a pitcher falls behind in the count, the called strike zone expands to give him a chance to come back. If an umpire is going to issue a four-pitch walk, the fourth of those balls has to clearly not touch any part of the zone. Conversely, when a batter falls behind in the count, the called zone tightens, avoiding a quick ring up and giving the plate appearance more of a chance to become competitive. Later in this article I examine another such case, in looking at home vs. away team strike zones.
My thought has been that maybe umpires have (even subconsciously) been relatively more lenient on left-handed hitters as a means of counteracting the growing defensive shift environment. To combat the large offensive advantage realized by left-handed batters every season, teams began implementing defensive shifts, initially directly targeting powerful left-handed batters. Teams shifted with left-handed hitters at the plate roughly five times as often as with right-handed hitters at the plate in 2010, for example. As a group whose calls have proven to balance the scales and keep people happy, umpires may have called the zone in such a way to prevent shifting from thwarting the league’s left-handed batters.
|Year||Strike Zone Size (sq. in)||K%||Shifts Against (BIP only)||BABIP||wOBA|
Consider BABIP by handedness split from the years in question. Between 2009 and 2013, when the right-handed zone was consistently just a handful of square inches larger, left-handed hitters averaged a whopping .006 higher BABIP across the league. As a response to this advantage, teams began shifting more regularly. In the three years since the relative zone size grew, from 2014 to 2016, right-handed hitters have now averaged a .001 higher BABIP.
Despite the smothering effect of the shift on left-handed batters, when we look at the weighted On Base Average (wOBA) gap between the two groups over the same periods, it has barely changed.
|Year Range||Strike Zone Size (sq. in)||K%||Shifts||BABIP||wOBA|
|2009 – 2013||6||-0.2%||-2795||-.006||.006|
|2014 – 2016||17||1.3%||-8577||.001||.004|
Perhaps this was just a coincidence, where constantly improving umpire evaluation tools and a marked effort to remove the “lefty strike” from the zone happened to be timed precisely when left-handed hitters needed the help, with strategic shifting putting a dent in their production. Of course, it could be argued that left-handed hitters shouldn’t be entitled to an offensive edge, but this is likely just a reflection of the platoon advantage they enjoy more often given that the majority of pitchers throw right-handed. Once again, whether on purpose or not, it seems that umpires may have aided in keeping one side of the game from gaining too much of an upper hand.
Home Is Where the Heart Is (and the Edges Too)
One strike zone effect that I have never covered is the difference in size based on whether the home team or away team is in the field. Given that umpires tend to err on the side of keeping people happy, it should be no surprise that the called strike zone is larger when the home team is pitching.
|Year||Strike Zone Size (sq. in)|
The home field advantage here is quite similar to the gap that used to exist between the zone for right-handed hitters and left-handed hitters, on the order of six to seven square inches. Importantly, the advantage has not shown any trend toward getting larger in the last few seasons, if anything tending towards being slightly smaller.
A worthwhile observation is that because the absolute difference in square inches between home and away team strike zones has stayed relatively flat over this stretch of years, but the size of the strike zone itself has been increasing, the home field advantage gained by the strike zone itself on a percentage basis has actually been reduced.
In the offseason between the 2010 and 2011, a new book called Scorecasting showed a home field advantage in major league baseball, in particular in high leverage situations. An article the same month estimated the strike zone was about 2.4 percent larger in general for home team pitchers.
Based on my calculations of the strike zone, that offseason marked a key point in time. Not only was it when the overall strike zone size started to noticeably grow, but it was when the home field advantage gained by the way the strike zone was called became less pronounced.
|Year Range||Strike Zone Size Difference (sq. in)||Strikes, but Called Balls||Balls, but Called Strikes||Total Advantageous Call Difference||W%|
|2007 – 2010||2.3%||-0.28%||0.38%||0.66%||.552|
|2011 – 2016||1.2%||-0.16%||0.28%||0.45%||.533|
I am not claiming that the strike zone difference explains all of the W-L record difference between these two eras, as it is quite a small difference. It is apparent, though, when looking deeper that despite only about half a percentage point advantage for the home team as a whole, the edge is certainly magnified when the game is on the line. To demonstrate this, I’ll examine the same game states as did this 2013 article by James Gentile.
|Year Range||Overall||9th Inning, 0 or 1 run game||9th inning, tie game||9th inning, tie game, RISP||9th inning, tie game, bases loaded|
|2007 – 2010||0.66%||1.11%||0.94%||3.71%||3.58%|
|2011 – 2016||0.45%||0.89%||1.58%||2.55%||4.02%|
|2007 – 2016||0.53%||0.98%||1.32%||3.01%||3.85%|
In the ninth inning of a close game, calls that favor the home team double in frequency. When the game is tied in the ninth and a runner is in scoring position, or even more so when the bases are loaded, such advantageous calls are six or seven times as likely! Unlike the apparent reduction of the strike zone advantage for the home nine overall since the start of the decade, the edge enjoyed in close, late game situations shows no clear trend toward disappearing.
Department of National Defense
It has to be nice to be a National League pitcher these days. One out of every nine hitters is practically an automatic out, half the teams in your league are “rebuilding,” and the strike zone you get to pitch to is sizably larger than that of your American League brethren.
|Year||Strike Zone Size (sq. in)|
In a general sense, it appears that the low and inside part of the strike zone for both left-handed and right-handed hitters was called more generously in National League parks than American League parks in 2016. Back in 2010 was the last time the National League zone was significantly larger, at that time clearly an inch lower than the American League zone. Once again, we see a correction starting in 2011, which seems to have been relaxed in the past two seasons.
The interesting thing about this gap is that starting in 2000, umpires have been assigned to games in both leagues throughout the same season. So there would be no reason to expect a systematic difference in the way the strike zone is called between the leagues. Perhaps this is merely random fluctuation based on how umpires with somewhat tighter and looser strike zones happen to have been scheduled, or a reflection of periods of catcher framing strength in the National League. For example, in 2015, Mark Ripperger umpired 10 more AL games than NL games, while Phil Cuzzi worked behind the plate for eight more NL games than AL games.
Whatever the cause, the strike zone for Andrew McCutchen this past season as a right-handed hitter in the NL would have been on average 18 square inches larger than the zone for Jackie Bradley Jr., a left-handed hitter of the same height in the AL. That’s equivalent to dealing with two baseballs being added onto the zone, as a ball is three inches in diameter!
The strike zone is the heart of the game. The home plate umpire, using guidance from the rule book and the umpire evaluation system feedback, plays a critical role in how the game is played. In this article I have shown several cases where the way the strike zone is called affects the game in a particular way.
To the credit of the umpires, when I calculated the percentages of pitches called balls that should have been strikes and vice versa, I did so against individual elliptical formulas I created for each batter hand and each season. Through this process, it provided another piece of evidence that umpire consistency has been improving during the PITCHf/x era, with particular improvements observable in 2008 and 2013, and 2016–the most consistent year yet. This is important to remember: Umpires have been improving behind the plate over the past decade.
If the rulebook strike zone definition is changed in the coming season or seasons, one aspect of the game that I would be curious to monitor is how quickly and consistently home plate umpires are able to call the bottom of the zone per the new rule. Certainly with post-game evaluations already in place, and umpires used to tuning their zones over the season based on this feedback, MLB is in a better place logistically to attempt a rulebook change than at any time in the past. The one disadvantage now is that every pitch is tracked by a system that provides data to the public, so people like me can actually see how well they are doing at sticking to their new rule. Thank you, MLBAM.
More importantly, it is clear that even without any rulebook changes, the strike zone has morphed in several different ways over the past decade, causing both direct and indirect effects on the game. I would suggest that a rule change to the strike zone definition has the potential to affect the game in ways we wouldn’t necessarily imagine. So depending where you fall on the spectrum of leaving the game the way it is versus trying new things in an effort to improve perceived areas of concern, keep an eye on any strike zone news over the offseason.
References & Resources
- All data courtesy of FanGraphs
- Special thanks to Sean Dolinar for providing shift data
- Jon Roegele, The Hardball Times Baseball Annual 2014, “The Strike Zone During the PITCHf/x Era”
- Tobias J. Moskowitz & Jon Wertheim, Scorecasting: The Hidden Influences Behind How Sports Are Played and Games Are Won
- J-Doug, Beyond the Box Score, “Strike Zone a Marginal Component of Home Field Advantage”
- Jon Roegele, The Hardball Times, “The 2015 Strike Zone”
- Brian Mills, The Hardball Times, “Are The Umpires At It Again?”
- Jayson Stark, ESPN.com, “Sources: Competition committee agrees to changes to strike zone, intentional walks”