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The World Series Baseballs Sure Seem Juiced. But Are They?

Just before the Houston Astros’ 13-12 victory over the Los Angeles Dodgers in Game 5 of the World Series, speculation was running rampant about juiced balls. Sports Illustrated’s Tom Verducci wrote an article detailing accusations from multiple pitchers that changes to the surfaces of the World Series baseballs were behind a spike of home runs in the Fall Classic.

This passed the eye test of even the most casual fan. Sunday’s game featured seven long balls, which wasn’t even the most in a game in this series. Game 2 saw eight homers, including five in extra innings — something that hadn’t occurred in any game, let alone a Series game. With one or two games left, the Dodgers and Astros have already broken the all-time marks for home runs in a World Series and in an entire postseason. The current rate of at-bats per home run stands at 15.8, which would slot in between the career marks of Albert Pujols and Frank Thomas. The average World Series rate since 1995 (the beginning of the wild-card era) is a much more pedestrian 34.8 at-bats per homer.

Examining the ratio of home runs per contact in all World Series since 1995, this year has the largest increase from the regular season to the Series.

The rate of long balls is up about 69 percent compared with the regular season, while the next largest bump is only 56 percent, achieved in 2009. According to a simple statistical test, the probability of witnessing so many home runs in the Series (considering the rate in the regular season) is about 1 in 80.1 That is a very fragile conclusion, however. Had we performed the same analysis after Saturday’s game, the result would be that the rate of home runs was not as elevated.2 We ought to be no more certain today that the balls are more lively than we were a few nights ago.

Despite the most recent round of allegations, there’s no compelling evidence that the baseballs are especially home-run prone this postseason — or any different than what we saw during the regular season. Since the middle of this year, evidence has been mounting that alterations to the baseball are to blame for the all-time high home-run rates. What we are seeing in this Series might be a result of the prior changes to the ball, rather than a whole new bout of modifications.

You won’t find a bigger believer in the juiced ball hypothesis than me, but the evidence for a World Series change driving a home run explosion is much thinner than the broader case that Ben Lindbergh, Mitchel Lichtman and I assembled over the course of the past few years.

For starters, the World Series spike is partially due to the fact that the Houston Astros are very good at hitting home runs. The team’s historically excellent offense is the source of many of the gaudy home-run totals already. So far, the Astros have hit 13 homers in five games, which sounds like it could be record-breaking on its own. But there were 103 five-game streaks in the regular season that saw a team drive 13 or more balls over the fence, including six such streaks by the Astros themselves.

There’s another problem with the juiced ball hypothesis. Verducci’s article alleges that the balls are more slick than normal. But academic research suggests that a more slippery baseball actually travels less far than a rough one. Similar to the dimples on a golf ball, a little bit of surface texture acts to reduce air resistance instead of slowing the ball down. So while we might expect slick baseballs to mess with a pitcher’s grip, we wouldn’t necessarily expect them to get out of the park more often.

There is some evidence to support the idea that the balls are more slippery. I used Statcast’s pitch tracking to compare the air resistance of playoff baseballs to those used in September and found a slight increase in drag in that time. The bump in drag coefficient of 0.023 is enough to reduce fly-ball distance by about 11.5 feet, which doesn’t explain at all why so many hitters are launching homers. Furthermore, this kind of fluctuation in air resistance happens all the time. (Drag coefficient dropped by a similar amount from August to September, for example.) As I detailed in an earlier article, whether because of manufacturing variations or intentional tampering, the drag on MLB baseballs often hops around from month to month, pulling home-run rates with it.

One way in which a slicker baseball could be causing more homers is if it is harming a pitcher’s ability to hold the ball. Some pitch types — those that require a more forceful grip on the baseball’s surface, like a slider — are expected to be affected most. Yet the evidence for an effect on that offering is much more murky. The ESPN Stats & Information Group found that pitchers’ spin rates and called-strike percentages dropped dramatically in the World Series, suggesting that they were having trouble getting a grip. But other sabermetricians have found no effect — or contradictory impacts on different hurlers. The increase in home-run rate might be coming from pitchers trying (and failing) to adjust to a new surface texture, rather than the ball itself being more conducive to homers.

Without more data — or a sudden wave of candidness from MLB commissioner Rob Manfred about any manufacturing changes to the ball — it’s hard to say for sure what’s happening to the ball. For now, we’re left with the possibility that this World Series is just business as usual in the home-run era — or that a new baseball is changing the game again.

Footnotes

  1. I used a binomial test, assuming that the rate of home runs per contact in the regular season is the true probability.

  2. The probability of seeing as many home runs as we saw up through Game 4 is about 1 in 20.

Rob Arthur is FiveThirtyEight’s baseball columnist and also writes about crime.

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