Thirty-eight percent of NPB’s home runs have disappeared, and anyone who tells you the ball isn’t the major factor, or that the reason is “the pitchers are better” is probably basing it on the games they watch without a broad sense of what the data is.

The wonderful thing about data is that if one gets enough of it, patterns can sometimes emerge that are not visible to even the most rigorous observer.

As Matt Murton said in 2013, when Japan’s previously soft mushy balls began flying better, “if it’s something that’s happening more or less across the board in both leagues, it’s not individual players doing something different.”

My podcast partner John E. Gibson (@JBWPodcast) has said he hasn’t seen balls that were really squared up and elevated dying, while he has seen balls that didn’t look like they had been struck that well go out. I saw a few of those this past week.

Home runs per ball in play March through May

Someone in the know

One other thing I did was talk to someone who works with a team’s TrackMan data and who has heard the explanations from both Nippon Professional Baseball and the baseball’s manufacturer Mizuno.

According to this analyst, both NPB and Mizuno have said that no intentional changes were made to the baseballs produced for use this season, but that they are definitely not flying as well as in past years.

“The best-hit balls are traveling about three meters less than in recent years,” he said. “They are looking for the cause but have yet to identify it.”

The most likely suspect is the yarn used to stitch the balls covers together.

“The stitching is becoming easily frayed,” he said.

This year’s problem may be analogous to MLB’s 2018 lively ball, that Dr. Meredith Wills (@Bbl_Astrophysics) discovered had been manufactured using yarn that was slightly thicker and stronger than what had been used in the past, resulting in some pitchers becoming extremely susceptible to blisters, and in the balls deforming less on contact with bats, making them more spherical and aerodynamically sound.

My best guess is that a supplier to a factory making baseballs delivered inferior yarn that was not up to spec and no one noticed.

There is word going around that NPB has begun introducing new balls that should conform to expectations. If so, they may still be using a mixed bag of balls, and it might not be until we compare July and August’s results with previous Julys and Augusts to get whether the balls are back to normal.

And while Major League Baseball appears to have been employing its version of the hidden ball trick, holding on to balls manufactured for previous years that are known to be livelier so that they can be employed in games where a player – or players – home run exploits have taken over the news instead of the balls other teams are using.

Wills documented this in 2022, when balls – identified by their numbering as being manufactured years earlier–were being hit into the stands at games in the second half of the season in which Aaron Judge was playing.

A friend who covers the Yomiuri Giants said it was highly unlikely NPB was doing that.

“You really think they’re that smart?” she asked.

That’s why what seems to be perpetrated is probably just another NPB being NPB.