Is the Baseball Actually Juiced?

Home runs are on the rise. We all know this. The number of homers per game is at an all-time high in 2019, and has increased by about 36% just since 2015:

Home Run Rate
Year HR/game
2015 1.01
2016 1.16
2017 1.26
2018 1.15
2019 1.37

What we do not know is exactly why.

Commissioner Manfred recently suggested that the current baseballs have less drag through the air, caused by the more perfect “centering of the pill” (the innermost part of the ball). It has basically become an operational fact that there is something going on with the baseballs. Manfred’s explanation implies that the flight of the baseball is the key difference.

To look at this closer, I considered the distance traveled by balls in the air as a function of the exit velocity and launch angle at contact. If the average distance on similarly struck balls has increased over time, it would suggest that the ball itself is more aerodynamically efficient.

Pitch-by-pitch data for the 2015-2019 seasons was collected from Baseball Savant via the Statcast Search page. Two random forest models were built for each year, one using all fly balls and one using home runs. To account for a possible difference in flight due to the warm air in the summer months, only data through June of each year was used. (At the end of the season, the analysis can be applied to the full data set). In both cases, the distance the ball traveled is the response variable and the exit velocity and launch angle are the explanatory variables. The models are applied to a test data set of various exit velocity/launch angle combinations.

The following plots display the predicted distance of the test points for fly balls by year.


Notice that the 2019 points (in dark blue) are not consistently higher than the other years. In fact, no particular year sees a consistently low or high prediction. The same is true for the home run models:

Similarly struck balls are not flying farther in 2019.

I am not a physicist, but I have to imagine that if the decreased drag of the baseball were the reason for the increase in homers, the results of this analysis would have been different. The balls hit at the same velocity and angle should travel a greater distance if they were cutting through the air more efficiently. Since this is not the case, it suggests that the “centering of the pill” is not driving the power surge. Whatever is happening, it seems like it is maybe happening at or before contact, not after.

Many pitchers have raised concerns over the seams of the baseball, and the inability to grip it. A difference of this kind would appear more plausible, as it could result in more pitching mistakes, and thus more balls hit out of the park. It would then not be that homers are sneaking over the wall where in previous years they would not have, but rather that hitters are more likely to square one up.

We hoped you liked reading Is the Baseball Actually Juiced? by Chris Russo!

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Daniel King
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Daniel King
scrap1ron
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scrap1ron

Has the percentage of barreled balls increased?

dbminn
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dbminn

I know you put in some work to prepare and review the data, but all you can report is that your particular analysis using batted ball events was inconclusive (a perfectly fine result). Testing MLB baseballs from different seasons is far more precise than sorting Statcast outcomes. For example, we know that surface roughness is an important determinant in the “carry” of a ball. You cannot throw physics out the window and assert it’s more likely hitters are squaring up more balls. The physical differences have been demonstrated and the physics used to conclude that the 2017 and 2019 balls… Read more »

Werthless
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Werthless

Is there a reason you chose random forests for this prediction? I typically will use random forests for categorical predictions, so I’m wondering if another regression or algorithm would produce different results for you.

Another method, if you cared to reuse the model you built, is to apply a model created with earlier year’s data and evaluate the residuals on different years. Does 2019 show positive, unexplained residuals? This allows you to provide summary stats that are quantifiable, versus the visual results you are displaying here (which may be influenced by binning decisions).

Dominikk85
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I did a similar analysis two years ago after the first homer spike and found also no increased distance on LA/EV combinations.

https://community.fangraphs.com/looking-for-evidence-of-a-change-to-the-ball/

Still think something must be with the ball. Sure more hitters now use an uppercut swing to increase LA and batspeed increasing techniques like weighted bats get more common but in 2018 LA and EV were up and HR/FB was down while 2017 EV was actually slightly down but homers up a ton.

Maybe the effect is at contact?

ScottyB
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Great article!