2020 Home Run Update: Is it the Ball?

Introduction

Most baseball fans know about the surge in home run hitting during the Statcast area, specifically starting with the 2017 season. Players are hitting home runs at a high rate and that is really changing the nature of run scoring in baseball. Although Major League Baseball is aware and concerned about the increasing home run rate, the reasons for this change aren’t quite so clear. One reason for the change in home run rates are changes in the manufactured baseball. Specifically, there is evidence that the ball in recent seasons is exhibiting less drag — that is, the ball is exhibiting more carry for given values of launch angle and exit velocity. Another possible reason for the increase in home run hitting is the change in the players’ launch conditions. That is, batters are hitting the ball harder and at more optimal launch angles and that is resulting in more home runs. The challenge is to better understand how these two factors are working together. For example, comparing 2019 and 2020, is the change in home run rate due to the ball effect or due to the players?

Here we’ll look at what is happening in the 2020 season — we have Statcast data for over half of the games played this season. We’ll use statistical methods that we’ve used before to explore the ball and player effects in hitting home runs.

Home Run Rates Remain High in 2020

First, let’s explore the pattern of home run rates. I define a home run rate to be the proportion of home runs hit among all balls put into play. Here I plot the home run rate for the seasons 2015 through 2020. (Here I am using 2020 data through the games of Friday, September 4.). What we see is that 2020 home run rate remains high, but it has slightly dropped off from the record home run rate in the 2019 season.

Is The High Rate Due to the Ball?

We first want to understand how home runs are related to launch conditions, specifically the launch angle and exit velocity off the bat. To do this, we fit a generalized additive model (GAM) which relates the probability of a home run to a smooth function of launch angle and exit velocity. If we fit this model to Statcast data for a particular season, we learn about the carry effect of the baseball. We fit this model separately for each season, so we have a “2015 model”, a “2016 model”, etc. which tells us about the ball effects for each of the five Statcast seasons 2015 through 2019.

Using these GAM models, we can answer the question — if we were using the ball from a particular season, say Season X, how many home runs would we predict in the 2020 season? Using the 2020 values of the launch variables, we can use the “Season X model” to construct a prediction interval for the count of 2020 home runs and see how the actual 2020 home run count compares to this prediction.

• If the Season X prediction is lower than the 2020 home run count, then the ball is livelier (less drag) in the 2020 season than in Season X.
• If the Season X prediction is higher than the 2020 home run count, then the ball is less lively (more drag) in the 2020 season than in Season X.
• If the Season X prediction agrees with the 2020 home run count, then the 2020 ball is similar to the Season X ball with respect to carry.

Below I have graphed 90% model predictions using boxes and the red line corresponds to the actual count of 2020 home runs through the games of September 4. What we see is the 2020 ball is somewhat consistent with the 2015 and 2016 balls, but it is clearly less lively than the 2017 and 2019 balls, but more lively than the 2018 ball.

Barrel Rates Continue to Rise

We now have a sense of the carry effect of the baseball in the 2020 season. What about the player effect? Statcast has recently defined a “barrel” as a suitably hard-hit ball that will likely result in good outcomes. Specifically, from the MLB site, one reads:

“The Barrel classification is assigned to batted-ball events whose comparable hit types (in terms of exit velocity and launch angle) have led to a minimum .500 batting average and 1.500 slugging percentage since Statcast was implemented Major League wide in 2015.”

I don’t know the exact formula used to classify a batted ball to be a barrel, but I can develop a similar formula from my modeling. Let’s consider a GAM model for the 2020 season where the probability of a home run is a smooth function of exit velocity and launch angle. Here’s a contour graph of the predictions from this model where the contour lines are drawn where the home run probability is equal to 0.1, 0.2, 0.3, …, 0.9. Note that 30 degrees appear to be an optimal angle for hitting a home run, although the range of suitable angles for hitting home runs increases as the exit velocity gets higher. In other words, the chance of a home run is more sensitive to the launch angle for lower exit velocities.

Using this model, I define a “barrel” as values of EV and LA where the home run probability (using the 2020 season model) exceeds 0.5. (I could use any cutoff value instead of 0.5 — I just wanted to use a value so the probability of a home run is high.). Using this definition, how has the barrel rate changed for batted balls in the Statcast era? Here is a plot of the ‘barrel’ rates. In contrast to the curious up and down pattern in the ball effects, the barrel rates show a steady increase over the Statcast period.

Wrapping Up

We see some clear patterns from our exploration of 2020 home run hitting:

• In-play home run rates remain very high in 2020, so home runs continue to be a major component of baseball offense.
• The ball effects on home run hitting have shown some interesting up and down patterns during the Statcast data. But the 2020 ball appears not to be as lively as the 2017 or 2019 balls. So the ball effect is clearly not driving the current high home run rates.
• But the players continue to hit balls hard and the the rate of of “good” or “barrel” batted balls has shown a consistent rise over the Statcast period. I didn’t have the definition of a MLB barrel handy, but the pattern of barrel rates using the Statcast definition would be similar to the pattern of the graph displayed above. The home run rate in 2020 is almost as high as the 2019 rate since the increasing barrel effect is compensating for the less lively baseball.
• If you want to read more about my work on home run hitting, here are some recent posts to look at:

What is Causing the Home Run Explosion?
Increase in Home Runs Due to Batter Behavior
Player HR Increase due to Modifications of Swing Behavior