Introduction

We have seen approximately half of the 2023 MLB season (I’m writing this on the morning of June 25). Given the volatility of home run production during the Statcast period (2015-2023), one wonders what is happening in this season. Are we seeing a lively ball as we saw in the 2019 season? Or will be the ball be as dead as we saw in the recent 2022 season?

As discussed in previous posts on this site, there seem to be two factors driving home run hitting. The first factor is the hitters — there is an increasing trend to hit balls hard (high exit velocities) at high launch angles (between 20-35 degrees) that produce home runs. Second, the drag coefficients of the ball have gone through changes. For example, in the 2022 season, the drag coefficients were high which means that balls didn’t carry as well as in previous seasons. This was a strong contributor to the low output of home runs in 2022.

This motivates the consideration of two rates:

• The rate of balls in play falling in different regions defined by launch speed and launch angle.
• The rate of home runs hit among the balls in play in different launch variable regions.

The first rate tells us something about the hard-hit/good launch angle characteristics of the hitters. The second rate tells us something about the carry properties of the manufactured baseball. We focus on these two rates to learn about home run hitting in the current 2023 season.

Home Run Hitting in the Statcast Era

The Shiny app HomeRunRates, displayed below, provides summary plots of these two rates over the range of dates April 1 through June 24. A couple of clear takeaways from the graph.

• Hitters continue to put high rates of balls in play where the launch angle is between 20-35 degrees and the exit velocity is between 95 and 110 mph. The 2023 BAIP rate of 10% in this region is the highest its been in the Statcast era.
• The home run rate of batted balls in this launch variable region is low in 2023, but not as low as the previous 2022 season. The 2023 HR rate is much lower than, say in the 2017 and 2019 seasons.

Comparing 2022 and 2023 Seasons

The Shiny app LogitHomeRates facilitates comparison of these rates for two seasons over the launch variable space. (Here we are using all Statcast data from the two seasons.) These comparisons are on the logit(p) = log(p / (1-p)) scale which facilitates comparisons of proportions controlling for the variability issue.

Here’s a snapshot of the display from this app. Here we have divided the launch variable space into 12 subregions. For each of the subregions, the top graph shows logit comparisons of 2022 and 2023 seasons of the balls in play rates and the bottom graph shows logit comparisons of the home run rates for the same two seasons. What do I see?

• The in-play rates at high launch speeds are consistently higher in 2023 than in 2022.
• The differences in the home run rates between the two seasons is less clear. It is interesting that the home run rates at high launch angles are lower in 2023, but the HR rates at angles between 20 and 30 degrees are higher. We might be looking at sampling variation here.

Comparing 2021 and 2023 Seasons

The snapshot below from the LogitHomeRuns app shows a similar comparison of logit rates for the 2021 and 2023 seasons.

• The balls in play rates look pretty similar for the two seasons, but there is a tendency for the 2023 rates to be higher.
• The home run rates in 2023 are clearly lower than the 2021 rates across all subregions of the launch variable space. The 2023 ball displays less carry than the ball used in the 2021 season.

Closing Comments

• (How Many Home Runs in 2023?). The current baseball does not appear to be as dead as the one used in the 2022 season. So I would predict the total 2023 home run count to be higher than 2022, but not as high as the 2021 season. (Baseball Reference stats tell us that 1.16 home runs are currently hit per team per game in 2023, compared with 1.22 (2021 season) and 1.07 (2022 season.).
• (R Notes) Both of these Shiny apps are available in my ShinyBaseball package. There is no need to install the entire R package. To run a particular Shiny app, all one needs to do is to download the specific app.R file, put the file is a separate folder, and run it in RStudio using the runApp() function. All of the data for the apps is read from my Github repository.