The changing of the guard at the MLB Office of the Commissioner prior to this season has prompted an openness to make adjustments to the game with an eye toward improving fan experience. Run scoring has been relatively depressed in recent seasons, with an expanding strike zone being one of the major culprits. There is an argument to be made that fans would prefer to see more runs scored, and so the league’s Competition Committee is monitoring the strike zone in 2015 for this reason.

When I checked in on the strike zone after just the first weeks of this season, its overall size was noticeably smaller than last year. In addition, while the bottom of the strike zone was as robust as it was during the initial weeks of the 2014 season, there was still room for it to grow to reach the level where it settled by the end the year.

Now two months into the season, the story has changed. The fact that the zone has been expanding downward since the start of spring training has been noted elsewhere, and these numbers paint the same picture.

Recent Strike Zone History

Year Strike Zone Size (sq. in) Strike Zone Size Below 21″ (sq. in) K% R/G 2009 435 0 18.0% 4.61 2010 436 6 18.5% 4.38 2011 448 11 18.6% 4.28 2012 456 19 19.8% 4.32 2013 459 30 19.9% 4.17 2014 475 47 20.4% 4.07 2015* 478 51 20.1% 4.19

The size and shape of the strike zone in 2015 has evolved to match very closely what was called in 2014. This would be expected, given that umpires are assessed on a nightly basis, their performance has been improving and becoming more consistent, and there was no indication from the league that umpires would be directed to call the zone any differently this season than last.

Perhaps the only surprising part about the first two months of the season has been that despite the strike zone filling out again, run scoring actually has increased. This is largely due to more hits being extra-base hits so far this season, with the league slashing .251/.315/.394 to date compared to last season’s final line of .251/.314/.386. It is also worth noting that at the end of May last season, teams had scored 4.17 runs per game, only to see offense deteriorate over the summer. So year-over-year scoring is, in fact, almost flat.

Here is a visualization of the strike zones to date in 2015, from the umpire’s perspective:

It’s been discussed at length that the strike zone for left-handed hitters is shifted toward the outside of the plate, while the strike zone for right-handed hitters is more or less symmetrical about the center of home plate. You also may have noticed in the strike zone plots that the overall size of the zone for right-handed hitters is larger than that of their left-handed hitting counterparts. This is an aspect I’d like to explore a little further.

To start, here is a recent history of the size of the strike zone by batter handedness:

Recent Strike Zone History, By Batter Handedness

Year Left-handed Hitters (sq. in) Right-handed Hitters (sq. in) Difference (sq. in) 2009 432 438 6 2010 434 439 5 2011 446 451 5 2012 453 460 7 2013 457 462 5 2014 467 484 17 2015* 472 484 12

While the zone for righties always has been slightly larger, since the start of last season that gap has noticeably widened. As a frame of reference, the diameter of a baseball is roughly 3.0 inches, which means its 2D area as it crosses the front plane of home plate is about 7.0 square inches. So the zone for right-handed hitters used to be larger by a little under the size of a baseball, but now the difference is more like two to two-and-a-half baseballs. The 2015 gap actually has closed a fair bit over the past 10 days, as it was looking more like the difference in the 2014 season until that point.

Here is a look at where the strike zone has changed between 2013 and 2014/2015 combined. Again this is from the umpire’s perspective. Green cells indicate areas that have become part of the strike zone, while red cells represent areas that have been dropped out of the strike zone.

While the above plots indicate very specific areas where changes in called pitches have led to cells crossing the threshold to be included in or excluded from the strike zone in the way I’ve defined it, a more complete pictures of recent developments can be gleaned from a heat map of changes across the entire plane at the front of home plate.

For all hitters, it is not surprising to see the majority of the growth at the expanding bottom of the strike zone. The wider gap in 2014 and 2015 between the right-handed hitter and left-handed hitter zones has come from two sources. First, pitches in the far bottom inside corner of the strike zone to right-handed hitters are now being called strikes more than pitches in the equivalent far bottom inside corner to left-handed hitters. Second, the reduction in the strikes in the high and outside regions of the strike zone has remained muted for right-handed hitters, while the trend is definitely stronger toward contraction of the zone in these same areas for left-handed hitters.

We can observe how walk and strikeout rates have trended over the last several years for right-handed hitters and left-handed hitters independently, as their respective strike zone sizes have changed differently.

Recent Statistics for (Right-handed Hitters – Left-handed Hitters)

Year Strike Zone Size (sq. in) BB% K% 2009 6 -1.8% -0.4% 2010 5 -1.7% -0.7% 2011 5 -1.0% 0.0% 2012 7 -1.5% -0.2% 2013 5 -1.2% 0.1% 2014 17 -1.4% 1.4% 2015* 12 -1.2% 1.3%

The strikeout rate difference between right-handed hitters and left-handed hitters jumped significantly in the same year that the theretofore-consistent gap in the strike zone size suddenly increased. In fact, the league strikeout rate for left-handed hitters actually peaked in 2012 at 19.9 percent and has fallen every season since (19.8 percent in 2013, 19.6 percent in 2014, and 19.3 percent to date in 2015).

Left-handed hitters have outperformed their right-handed hitting counterparts in both OBP and OPS in each of the seasons in the table. By having the platoon advantage more often, being a step closer to first base by virtue of their starting point in the batter’s box, and now an even smaller relative strike zone with which to contend, left-handed hitters certainly have some built-in advantages.

If the strike zone really is to be reviewed, and umpires are instructed to call a different size and/or shape of the strike zone at some point in the coming seasons, and be assessed based on that, it would seem prudent to consider the difference in the zones for batters from each side. After a number of years in which both the difference in relative size of the strike zone and the strikeout rate itself for left-handed hitters and right-handed hitters were almost identical, the strike zone has morphed in a way where it is less equitable to right-handed hitters, and they are now striking out at a higher rate than left-handed batters.

References & Resources