Home Run Explosion?

If you have been following baseball in the current 2019 season, you probably have noticed that home runs seem to be a big story. For example, Dodgers matched the MLB record by homering in 32 consecutive home games. Given all of the recent discussion about home runs, that raises the question — are home runs indeed up this season?

Outcomes During a Plate Appearance

Before we discuss home runs, let’s talk first about the changing pattern of outcomes during a plate appearance. Below I have graphed the rate of strikeouts, the rate of walks, and the rate of balls in play for the last 5 seasons — I am using data through yesterday’s games (April 17, 2019). Strikeout rates are continuing to rise. Walk rates stabilized from 2017 to 2018 but they are up again in 2019. Also the rate of balls in play is in steady decline — from about 70% in 2015 to about 65% in 2019.

How does this relate to home runs? Well, players are putting fewer balls in play so there are fewer opportunities to hit home runs.

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In-Play Home Run Rates

I was part of a MLB commission that was exploring reasons for the big increase in the home run rate from 2015 to 2017. But, interestingly after we completed our study, the home run rate (defined here by the percentage of home runs among balls put in play) dropped in 2018. But the home run rate has taken a big step up in 2019.

Home Run Rates Per Plate Appearance

Okay, so there are fewer opportunities to hit home runs (fewer balls put in play), but the rate of in-play home runs has increased. What about the rate of home runs per plate appearance, defined by 100 * HR / PA? As displayed below, this home run rate has also increased –the 2019 value is currently about 3.4 % which is the highest in baseball history.

Let’s Predict

I won’t do a full prediction analysis, but we can make a quick prediction of the total home run count in 2019.

Currently, we have seen 700 home runs in 270 games in the 2019 season. Suppose that home runs will continue to be hit in 2019 at the same rate of 700 / 270 = 2.59 home runs per game. There is a total of 2430 games played in the season. With a simple calculation, we’d predict that

2.59 * 2430 = 6294

home runs would be hit this season — this is a big increase over the current record of 6105 home runs hit in 2017.

Some Comments

There are many variables that influence the home run count for a particular season. I’ll mention a few of them here and they deserve additional exploration.

1. Weather? Generally, there is a temperature effect — there tends to be fewer home runs hit in colder weather. So maybe the prediction of 6294 home runs is conservative since many games have been playing in cold weather.
2. Off the bats measurements? Home runs are associated with balls that are hit hard (high launch speed) and a good launch angle. It is possible that batters are hitting at higher launch speeds, although my sense is that this is a small contributing factor to the increase in home runs.
3. Carry? One reason for the big increase in home run hitting, say from 2016 to 2017, was that balls tended to carry further given particular launch speed and launch angle measurements. That might account for what’s happening this season, although I haven’t explored that yet. This issue about carry is really about the characteristics of the baseballs that are used by MLB.
4. Other variables? Pitchers are pitching at higher velocities — this might contribute to more home runs. Even variables like the count might be a factor. Batters tend to hit home runs on favorable counts (3-1, 2-0, etc) and if they get more of these favorable count opportunities, that would lead to more home runs.