Anomalies in the parks

As some of you probably noticed last season, 2021 was a weird year, and not just because two teams that hadn’t finished as high as fifth since 2018 won championships, but because it looked — in the case of Koshien Stadium and Jingu Stadium, that what we knew to be true no longer was.

In recent years, the spectrum of how much the main parks in Japan’s Central League influence offense has been pretty stable from most pitcher friendly to least:

  1. Nagoya (Vantelin) Dome, Chunichi Dragons
  2. Koshien Stadium, Hanshin Tigers
  3. Mazda Stadium, Hiroshima Carp
  4. Tokyo Dome, Yomiuri Giants
  5. Yokohama Stadium, DeNA BayStars
  6. Jingu Stadium, Yakult Swallows

Tokyo Dome has probably been the closest thing to a neutral park in the league, because it suppresses everything but home runs, which are pretty easy there.

What was weird in 2021 was that compared to the other five parks in the league, Koshien increased home runs and runs scored, while Jingu and Tokyo Dome switched places, with Jingu being essentially a neutral park with easy home runs, while Tokyo Dome increased scoring by nearly 40 percent.

The biggest factors at play were probably the weather patterns around Koshien and Jingu last season, although one thing that worked against the Tigers, who allowed runs 14 percent more often in their home games in 2021, was having their best pitcher, Koyo Aoyagi, throw the bulk of his innings on the road, and then pitch less well at home.

The first indication that something might be wacky was in the middle of the season, when my podcast partner, John E. Gibson (@JBWPodcast) mentioned that the Swallows’ team ERA was nearly the best in the CL

The quality of the Swallows’ pitching in 2021 completely snuck up on me, because a) they’d been pretty bad for a long time despite indications of a bullpen renaissance the year before, and b) the Swallows play in a park that since 2016 has inflated their home game ERAs by 0.48 a year. In 2021, it was half that with a 3.58 home ERA and 3.37 in road games.

Another possibility is that before during and after the Olympics, in the hottest part of the summer, when much of Japan turns into a humid sauna — or as the Tokyo Olympic bid committee called when trying to secure the game in 2013 “pleasant and mild” — the Swallows and BayStars were evicted from their home parks when offenses are boosted by the heat.

Playing in a more favorable environment didn’t make the Swallows pitching better, but did make it LOOK better than it already was. This is a distinction that is frequently lost when discussions of what parks are best for individual players turn into discussions about what parks are better for individual players’ performance stats — as opposed to value.

The second most annoying discussion relating to park effects is the belief that park effects are created by players.

My two favorite examples of this are:

  • One colleague over the past few decades explaining to me that Yokohama Stadium appeared to be a good home run park because the BayStars pitching was so bad.
  • The late great Wayne Graczyk predicting that the Hiroshima Carp would hit more home runs after moving from bandbox Hiroshima Shimin Stadium to larger Mazda Stadium, because “the players will no longer be trying to hit home runs.”

Here are how I estimate the 12 main league parks stacked up against each other in 2021.

Because each season is a small sample size, I use data from the two years before and after to smooth it out, so while Jingu appears to have played as a neutral park in 2021, I’m not going to overlook the mountain of evidence that says it really isn’t.

A number of 1.00 indicates a neutral park, with less than 1.00 favoring the pitcher:

Central League

StadiumRun factorHome run factor
Vantelin Dome.866.623
Mazda Stadium.949.898
Koshien Stadium.980.999
Yokohama Stadium1.051.16
Jingu Stadium1.091.64
Tokyo Dome1.281.53

Pacific League

StadiumRun factorHome run factor
Kyocera Dome.920.922
Rakuten Seimei.963.937
Casa de Pepe1.031.39
Sapporo Dome1.03.837
MetLife Dome1.061.03
Zozo Marine1.211.41

Park adjustments

Here are the up-to-date park run and home run adjustments for all 12 NPB teams as best as I can figure them for 2021. Although I don’t adjust for opponent quality, there is a tiny difference in the offensive and defensive adjustments. This comes from the different totals of outs made on offense in each park and the outs recorded on defense at the same venue.

These are not individual park factors, but the aggregate effect of the effects on runs and home runs in each team’s innings at bat and in the field over the course of the season.

What this means is that home runs were about 16 percent more likely in Yakult Swallows games than the Central League average in 2021, a low figure that off-hand I’m guessing resulted from the Swallows being evicted from Jingu Stadium during the hottest months of the summer because of the Tokyo Olympics.


TeamO R AdjD R AdjO HR AdjD HR Adj
Nippon Ham1.0151.0160.9470.942

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