Home runs are (relatively) down in 2020

The makeup of the baseball is the perfect antithesis to Mike Trout. Trout — he’s consistent. The baseball? Not so much.

In each of the last four years, we’ve seen wild fluctuations in the aerodynamics of the baseball. Beginning in the second half of 2015, the ball began to fly farther. In four full seasons since, we have seen nothing short of a barrage of long balls. Each season from 2016 to 2019 ranks among the top five in homer-happiest seasons in baseball history, with 2019 blowing away the homer record. Last year alone, four teams — the Twins (307), Yankees (306), Astros (288) and Dodgers (279) — all surpassed the single-season record for most homers by a team in one year. The home run has pretty clearly overtaken baseball.

Even though we’ve been drowning in homers, there has been some wild year-to-year variation. Last year, for example, saw 1,191 more homers than the year prior. The story has become less about the juiced ball and more about how the ball varied so wildly on a year-to-year basis.

And that variability doesn’t appear to be letting up, either. Earlier this month, Rob Arthur published a piece at Baseball Prospectus showing that the 2020 baseball is different yet again. Drag levels on this version of the baseball are more similar to the 2018 version than the 2019 version, which Arthur explained would yield roughly a 10 percent decrease in home runs year-over-year.

The early returns do show a decrease in home runs that is unlikely to be explained by chance alone.

Upon first glance, this might not be immediately evident. Into games on Wednesday, the percent of batted balls resulting in home runs this year is 5.1 percent, a rate that is slightly lower than 2019 (5.4 percent). That rate is, however, higher than both 2018 (4.4 percent) and 2017 (4.8 percent). But, starting a season in late-July adds an extra confound that wasn’t the case in any of the three prior seasons, as warm weather contributes to an increased home run rate. Here’s the data showing this trend, from 2019:

Though slight, there is an increase in home runs as the baseball season makes its way into the summer. So, of course, it’s not a direct apples-to-apples comparison of 2020 to prior years. We need to focus on the timeframe of the start to this season, the late-July into early-August span. And here it is, a comparison of home runs as a percentage of batted balls specifically from July 23 to August 11 in each of the last four years:

From this, it’s pretty evident that home runs are down, at least slightly. It’s highly unlikely that this is due to random variance. A two-sided test for proportions does demonstrate that this change is statistically significant at the 0.05 alpha level (p-value of .02). A 95-percent confidence interval for the home run rate during this time period is 4.7 to 5.5 percent — suggesting that the 2018 home run rate of 4.6 percent is also outside the interval.

Ultimately, while the home run rate certainly seems like it will be lower in 2020 than 2019, it remains to be seen where exactly it will fit in the context of the last four seasons. Another important metric to analyze here is home runs as a percentage of all barrels, and 2020 once again comes in lower than the prior seasons.

Here, we see that only 56 percent of barrels are home runs in 2020, down seven percentage points from last season and down three percentage points from 2018 as well. Also curious is the fact that, while barrels had been on a steady increase from 2016 to 2018, before exploding in 2019, they are also down a touch this year overall. This might be more indicative of a larger offensive outage right now, one that Eno Sarris attempted to explain in his piece over at The Athletic earlier this week. Also interesting in this regard is the fact that the total number of batted balls is also down, likely due to a combination of the numerous postponements due to COVID-19 and the extraordinarily high strikeout and walk rates thus far.

The data, however, does not necessarily show that batted balls aren’t traveling as far. In both 2019 and 2020, approximately 70 percent of all home runs are hit between exit velocities of 99 and 110 mph and launch angles of 21 and 36 degrees. When evaluating all batted balls — not just home runs — in this range, we find that the average distance has only fallen by two feet year-over-year, from 398 to 396 feet. A much more notable difference is clear when considering the 2018 distance on these types of batted ball events: That year, they went only 392 feet.

There are so many factors that could be at play here. While I don’t doubt that the balls are different — Arthur has the exact drag coefficients, so that’s pretty clear — we’ve seen a host of changes this year that may contribute to these results. The move from Trackman to Hawk-Eye for Statcast deserves a mention, as does the introduction of humidors in Fenway Park, Citi Field and T-Mobile Park.

On the whole, it’s pretty clear that we’re seeing at least a small reversal in the explosion of home runs witnessed in 2019. Balls are still leaving the yard often, but in this sample to start the season, the home run-per-batted ball rate is about nine percent lower year-over-year.

Devan Fink is a sophomore at Dartmouth College and a Contributor at Beyond The Box Score. Previous work of his can be found at FanGraphs and his own personal blog, Cover Those Bases. You can follow him on Twitter @DevanFink.