Examining the Increasing Correlation of Home Run Rate and Strikeout Rate

Examining the Increasing Correlation of Home Run Rate and Strikeout Rate

For the first time in baseball history, Home Run Rate and Strikeout Rate had a positive correlation in 2022. 1 That’s probably not a surprise to anyone who keeps up with Major League Baseball. But, it’s worth examining where we are, how we got here and where we might go. 

The league had a 15.8% SO Rate in 1968. After that season the mound was lowered and the size of the strike zone was decreased. These changes led to Major League hitters striking out at a 15.2% rate in 1969 and a steady decrease in SO Rate, down to 12.5% in 1980. The average MLB SO Rate for the seasons from 1969 through 2014 is 15.9%. The same figure for the 2015-2023 seasons is 22.5%. How did that happen? 

One of the primary (and most obvious) reasons for the increase in strikeouts is an increase in pitch velocity. The average four-seam fastball thrown in the 2008 season was 91.9 MPH, the same ‘average’ in 2023 was 94.2 MPH. But, it’s not just fastballs, sliders gained an average of 1.8 MPH in the same span (Cooper, 2023). Again, this won’t shock anyone who pays attention to MLB. The velocity of every pitch is shown on-screen and in the ballpark, and pitchers keep getting injured at an alarming rate. 

Big league pitchers have paired better ‘stuff’ with their gains in velocity. In 2019, 88 MLB pitchers threw at least 10 four-seam fastballs with greater than 2500 RPM. The 2021 season produced 119 such pitchers. Hurlers have also started throwing more fastballs in the top of the zone (roark, May 2021), where the ‘ride’ that backspin on a fastball produces is most useful to pitchers. So, the typical Major League pitch travels faster, is tougher to identify and is better located. Yeah, batters are striking out more, I don’t blame them. 

The advances in pitch velocity and stuff have led to many MLB pitchers becoming more comfortable living in the strike zone with their fastball. From 2015-2021, Chase Rate went down each season but Swinging Strike Percentage increased each season. Additionally, the Swinging Strike Rate increased for fastballs in every count during our 2015-2021 window (Ciardiello, 2021). Additional research shows that Swinging Strike Rate increased from 8.6% to 10.4% in only 5 years (2017-2021). Hitters have adjusted, but many have chosen an approach that is less contact-oriented. 

From 1955 to 1968, Strikeout Rate increased from 11.4% to 15.8% and the league’s ERA dropped by over a run. In 2008 pitchers struck out 6.8 batters per 9 innings and had a 4.32 ERA. By 2019 K/9 shot up to 8.9 and the collective MLB ERA only dropped to 4.31. MLB batters swapped contact for getting on-base and/or hitting for more power (Baccelliari, 2019). Even though the average batter strikes out more often than they used to, they’ve still become more productive with 2 strikes:

(Ciardiello, 2021)

Hitters managing to maintain overall production despite putting fewer balls in play brings me to my other topic. 

Sticking with our starting point of when the mound was lowered and the size of the strike zone decreased, the HR Rate was 1.82% in 1968 and jumped all the way to 2.36% in 1969. There has only been 1 season since that time where the HR Rate was lower than 1.82% (1976, 1.68%). Lowering the mound clearly had an effect on the frequency of Home Runs, but 2.36% up to the high point of 4.04% in 2019 still leaves a lot to explain. 

We’ve covered that professional hitters are trying to do more with less by hitting more HRs and sacrificing some base hits in the process. It’s not a surprise that if the best hitters in the world try to hit more HRs, they’ll hit more HRs. It’s hard to blame batters for getting deeper into counts (and striking out more) when data suggests they should swing less and focus on pitches where they do the most damage. 

The actual baseball played a role in more Home Runs from 2015-2020. It’s not a secret that MLB changed the ball midway through the 2015 season and the numbers tell the same story. From 2014 to 2019, HR Rate increased every season except 2018. Even after that setback, the 2019 HR Rate is the highest in our sample (4.04%). 2  Looking at the increase from the player level, in 2021 there were 5 players that hit 40 HR and 43 players that hit 30 HR. 2019 saw 10 players eclipse 40 HR and 58 players accounted for 30 or more HR. If you’re not a fan of this trend, the good news is that MLB introduced an even newer ball in 2021 that seems to produce higher Exit Velocities but not carry as far (Fink, April 26, 2021). 

Maybe the biggest reason for the increase in how often MLB players hit Home Runs is expansion and the resulting increase in the number of pitchers in the league. 

MLB HR Rate Before, During & After Expansion Seasons

(Expansion Seasons in yellow)

These 4 expansion seasons account for a 1.88% hike in league HR Rate and, in 3 of the 4 scenarios above, the rate climbed even higher within 2 seasons following expansion. The explanation for this is simple, since Major League pitchers are generally the best in the world, adding more pitchers to that pool brings the average skill-level down and gives hitters more chances to hit Home Runs. 287 MLB players pitched in 1968 and 864 pitched in 2023. That’s more than a 200% jump in the number of pitchers used and HR Rate increased by over 94% during that time. 

Now that we’ve explored many of the reasons that SO Rate and Home Run Rate have risen to all-time highs in recent seasons, we can see a clear pattern emerge. The gains made in pitch velocity and stuff have led to pitchers throwing more fastballs that are harder to hit in the strike zone. On the flip-side of that, hitters are waiting for pitches they can do more damage with and trading contact for more power. Given that the newest (2021) version of the baseball seems to reward fly balls but produce worse results on line drives, I wouldn’t expect hitters or pitchers to change their general approach until another change in the baseball or a relative improvement in the skill-level of MLB batters forces one of the groups to rethink their strategy. There are certainly other factors at play, including the implementation of Pitchf/x to grade umpires and new rules regarding ‘sticky stuff’. However, the primary reasons for HR Rate and SO Rate becoming increasingly linked are that pitchers have gotten better and hitters have found a way to be just as productive, even if that production takes a different shape. 

1 – HR Rate calculated as HR/(AB+SF)
SO Rate calculated as SO/PA
Code used to calculate correlation here
Correlation Table can be found here

2 –  HR, AB, SF & SO Rate are from Fangraphs.com

Bibliography

1. Baccelliari, Emma (May 15, 2019). Explaining MLB’s Monumental Jump in Strikeouts This Year. Sports Illustrated. https://www.si.com/mlb/2019/05/15/strikeout-record-pace-best-pitchers
2. Ciardiello, Carmen (May 17, 2021). Let’s Take Another Stab at Unpacking the Rising Strikeout Rate. Fangraphs. https://blogs.fangraphs.com/lets-take-another-stab-at-unpacking-the-rising-strikeout-rate/
3. Cooper, J.J. (September 27, 2023). Ever-Climbing Velocity Pushes Hitters to the Brink. Baseball America. https://www.baseballamerica.com/stories/cooper-ever-climbing-velocity-pushes-hitters-to-the-brink/
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6. Fink, Devan (October 5, 2021). There Were Fewer Homers this Year, but the Long Ball Still Reigns Supreme. Fangraphs. https://blogs.fangraphs.com/there-were-fewer-homers-this-year-but-the-long-ball-still-reigns-supreme/
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