There’s little doubt a gap exists between the Pacific and Central leagues in terms of quality, based on interleague results since 2005.

This is the second of three pieces I’m doing on the differences in quality between Japan’s two leagues.

Last time, I took up the issue of how the pitching in the two leagues differs now, and evaluated Alex Ramirez’s idea that the PL is a harder-throwing league and that the CL needs to do a better job of drafting and developing hard throwers.

This time I want to replicate a study I did a few years ago to evaluate how much of the PL’s advantage is on the pitching side. While I agree with Ramirez that PL hitters are better because they are used to seeing better pitching, this study suggests that the league’s competitive edge started in the batter’s box.

In the final piece, I’ll use my draft database to evaluate the quality of domestic player development by both leagues.

The study

A few years ago, when the PL’s superiority began to fill our rear-view mirrors like a tail-gating monster truck, I derived a study to figure out where the league’s advantage came from.

If we look at how well each team does against visitors from both leagues in their main stadiums, we can control for park and talent. If we limit the time frame to games prior to July — since interleague play runs from May to June — we can eliminate the noise from league games played in the year’s hottest months.

The study uses runs per nine innings and also wOBA, and takes a weighted average of performance against league and interleague opponents. If the PL pitching is superior we would expect offenses from both leagues will do more damage against visiting CL pitchers than visiting PL pitchers. If the PL hitters are superior we would expect visiting PL batters to do better than CL visitors.

The study suggests these conclusions:

• The belief that the difference between the PL and CL is mostly related to pitching is unsupported.
• CL pitching was probably a little better from 2005 to 2012, but is no longer as good.
• PL pitchers used to struggle in the CL’s home-run friendly parks

The pitchers

I’m going to measure the quality of each league’s pitching by looking at the weighted league averages of: runs allowed per nine innings by visitors and home opponents’ basic wOBA.

2005-2012

These figures support Trey Hillman’s 2006 after the first round of interleague play that the CL was the better-pitching, harder-throwing league. But that was then and this is now. Here are the weighted averages since 2013.

The gap actually might not be as large as the tables above indicate, as I’ll go into below. It seems that size does matter when it comes to ballparks.

2013-2019

Since 2013, whatever advantage CL pitchers might have had over their PL counterparts has evaporated, and as Ramirez suggested, the PL pitching (and defense) is definitely better than what passes for good enough in the CL.

The hitters

We’re going to flip this around and now look at weighted league averages against CL and PL visitors in home pitchers’ RA9 and visitors basic wOBA.

2005-2012

2013-2019

So if the PL advantage in interleague play has been primarily on the pitching side, then somebody better tell their hitters that. I did a parallel wOBA study that removed pitchers and designated hitters from the equation, but it made little difference.

The big and small of it

The one area in the study where PL pitching was inferior when measured by both visitors’ runs allowed per nine innings and opposing home teams’ wOBA was in the CL home parks from 2005 to 2012.

Former Swallows pitcher Shohei Tateyama thought that the PL was better at developing pitchers because their big parks were more forgiving. Before Fukuoka’s Home run terrace and the new shorter dimensions in Sendai and Chiba’s Home run lagoon, all six PL parks were tougher home run parks than Tokyo Dome, Jingu and Yokohama — and Hiroshima Shimin until 2008.

I broke them down into these four small parks, and three large parks, Koshien Stadium, Nagoya Dome and Mazda Stadium — more neutral than large, really. Here are the weighted averages of RA9 by visitors from each league since 2005 in the months prior to July.

That’s not a lot of information but it’s about what I expected. The steady proliferation of with three downsized PL parks would make a further breakdown since 2013 difficult and there are times when I’m adverse to work.

Conclusion

So that’s the footprint of the PL’s current advantage. No matter which league you play in, both leagues’ batters and pitchers playing in their home park put up better numbers when the visitors are from the CL than when they are PL teams in interleague.

When I ask people why the PL is better, the standard answer is pitching, perhaps because it’s easier to see how it could be better. But from here it simply looks like the PL pitching and hitting developed in tandem — starting with the hitting.

Next time, I’ll get into the makeup of the talent in the two leagues, and how talent has flowed into them and between them.