The Playoff Strike Zone

Watching the Yankees and Twins playoff game last week, I noticed that it seemed the strike zone was pretty tight. This was just one playoff game and a few pitches, but I thought of the possibility of a shrinking strike zone in the playoffs, as umpires may be less forgiving.

Baseball Savant would not let me run a query for all seasons, so I divided my data queries into three-year increments, dating back to 2009. First, for each three-year set, I found the total called strikes and balls (pitches not swung at) and calculated what percentage were called strikes for the regular season and the playoffs.

Years	Regular Season	Postseason
2015-17	33.48%	34.31%
2012-14	34.43%	33.64%
2009-11	33.69%	34.7%

The zone grew a little from the regular season to the playoffs in 2015-17 and 2009-11, but shrunk a little in 2012-2014. So no indication of any sort consistent strike-zone change.

Using the Statcast detailed strike zones, I looked at the same called-strike rate on zones 11, 13, 17, and 19. These zones are the corners of the strike zone, with half of the area in the zone and half outside of it.

Years	Regular Season	Postseason
2015-17	22.45%	25.27%
2012-14	20.87%	24.48%
2009-11	18.40%	20.98%

There is a clear change here, and not in the direction I thought it would. There is a 3% average increase of called strikes on the corners here. Not a huge change, but certainly a difference-maker. Looking at just this season, the called-strike rate on the corners has increased from 21.88% in the regular season to 24.14% in the postseason.

Let’s look at the rest of the edges of the strike zone (area between the corners).

Years	Regular Season	Postseason
2015-17	55.19%	56.30%
2012-14	54.90%	56.68%
2009-11	53.18%	55.04%

Not nearly the difference of the corners, but an average of a 1.58% increase. There is definitely a trend of strike-zone expansion.

This also begs the question of whether Rob Manfred’s initiative to raise the bottom of the strike zone has had any effect. There was worry that umpires had become too trigger-happy with pitches around the knees. Looking at only pitches in the bottom edge of the strike zone, the called strike rate in 2014-16 for the regular season was 34.78%. In the 2017 regular season, it was 33.85%. So it appears there was definite change. But has Manfred’s wish stayed consistent in the playoff?

The called-strike rate in the zones bordering the bottom of the strike zone (through October 9) is 37.38%, nearly four percentage points higher than the regular season. So umpires are not meeting Manfred’s hopes in the playoffs thus far.

We can’t be sure of how this affects playoff games, but Dustin Pedroia struck out looking on a bottom-corner pitch barely in the zone with the bases loaded. Maybe if it were like the regular-season strike zone, we would be talking about an Astros and Red Sox Game 5. Obviously we cannot assume anything, but it’s hard to argue this does not change the game at least a little.

Clearly, my initial inclinations watching the Yankees and Twins play were wrong. I was surprised to discover the zone actually expands, not shrinks, in the postseason. In a climate that is already more difficult for hitters, as most teams are pitching their best stuff, it is only making postseason hitting even more difficult.