Examining Home Run Rates by Ballpark
At the beginning of May, I wrote two articles about the slightly-deadened baseball’s effect on league-wide home run rates. The conclusion was pretty much exactly what you’d expect: A bouncier ball with more drag did reduce home runs, particularly among softer-hit balls at lower launch angles. In 2019, these events were the wall scrapers that barely went out of the yard. In 2021, these events are now doubles and outs, with the increase in fly outs likely contributing (at least somewhat) to baseball’s diminished run environment overall.
There were a handful of outstanding questions that I still had, one of which was the impact of the new baseball on a ballpark-by-ballpark basis. Though league-wide trends are certainly an interesting and informative way to see the effects of a new baseball on run scoring, it is also important to examine in which parks hitters are having a more difficult time getting the ball into the seats. That allows us to understand better how park effects may have been altered to different degrees as a result of MLB’s switch to the new baseball.
But it’s not just the baseball that is contributing here. MLB reportedly added humidors to five stadiums for the 2021 season, bringing the total league-wide to 10. The Rockies, Diamondbacks, Mariners, Mets, and Red Sox already had humidors in their stadiums pre-2021, but which five teams are new to that list has yet to be disclosed. We can only guess which parks now have them, but it is important to keep in mind that the ball is not the only difference.
Also important to remember when looking at ballpark-level data: The players on the home team make a huge difference in determining home run rates. It’s entirely possible that, between 2019 and ’21, a team added home run hitters to its lineup or acquired home run-adverse pitchers for its staff, or the opposite could also be true. To mitigate these effects, I only analyzed a specific slice of fly balls: those hit at an exit velocity at or above 95.0 mph, at an exit velocity below 110.0 mph, and at a launch angle below 30 degrees — the very fly balls most impacted by the new baseball in my prior analysis. I also only included fly balls hit in games on or before May 31 to control for weather effects. (That is why I am comparing 2019 to ’21.)
This is a fairly large range that is impacted by quality of hitters and pitchers. Better hitters might be hitting more 29-degree, 109.9-mph fly balls, and worse hitters could be hitting more 23-degree, 95.1-mph fly balls. I compared the samples of 95–109 mph, less-than-30 degree data from 2019 to ’21 to evaluate just how pronounced these effects were and found that average exit velocity did not shift by more than 1 mph and that average launch angles did not shift by more than 1 degree. For transparency’s sake, here are all 28 parks’ average exit velocities and average launch angles on fly balls within this range in both 2019 and ’21, excluding the Blue Jays and Rangers, who played in different stadiums in 2019 than they do currently.
Park | Team | 2019 EV | 2021 EV | EV Diff | 2019 LA | 2021 LA | LA Diff |
---|---|---|---|---|---|---|---|
Chase Field | ARI | 101.4 | 100.8 | -0.5 | 26.6 | 26.7 | 0.1 |
Truist Park | ATL | 102.5 | 102.6 | 0.1 | 25.8 | 25.6 | -0.2 |
Camden Yards | BAL | 102.1 | 102.4 | 0.3 | 25.4 | 26.4 | 1.0 |
Fenway Park | BOS | 102.1 | 101.3 | -0.8 | 25.3 | 25.5 | 0.3 |
Wrigley Field | CHC | 102.9 | 101.9 | -1.1 | 25.6 | 25.7 | 0.1 |
Great American Ballpark | CIN | 100.8 | 102.0 | 1.1 | 26.4 | 26.1 | -0.3 |
Progressive Field | CLE | 102.9 | 103.1 | 0.2 | 25.8 | 25.5 | -0.3 |
Coors Field | COL | 102.5 | 101.6 | -0.9 | 25.1 | 25.1 | -0.0 |
Guaranteed Rate Field | CWS | 102.9 | 101.7 | -1.2 | 24.9 | 25.7 | 0.8 |
Comerica Park | DET | 102.6 | 102.0 | -0.6 | 25.8 | 25.8 | 0.0 |
Minute Maid Park | HOU | 101.7 | 101.2 | -0.5 | 25.7 | 26.1 | 0.4 |
Kauffman Stadium | KCR | 102.9 | 102.1 | -0.8 | 26.1 | 25.7 | -0.3 |
Angels Stadium | LAA | 101.7 | 103.2 | 1.5 | 26.1 | 26.1 | 0.0 |
Dodger Stadium | LAD | 101.6 | 101.5 | -0.1 | 25.7 | 25.9 | 0.1 |
loanDepot Park | MIA | 101.3 | 101.8 | 0.4 | 25.6 | 25.3 | -0.4 |
American Family Field | MIL | 101.9 | 101.6 | -0.4 | 25.6 | 25.7 | 0.1 |
Target Field | MIN | 102.5 | 102.9 | 0.5 | 26.4 | 26.0 | -0.4 |
Citi Field | NYM | 101.3 | 101.3 | -0.0 | 25.9 | 25.7 | -0.2 |
Yankee Stadium | NYY | 102.4 | 102.1 | -0.3 | 25.5 | 25.8 | 0.2 |
Oakland Coliseum | OAK | 102.2 | 101.4 | -0.8 | 26.0 | 25.8 | -0.2 |
Citizens Bank Park | PHI | 102.6 | 102.4 | -0.3 | 25.7 | 25.8 | 0.1 |
PNC Park | PIT | 101.7 | 101.8 | 0.1 | 25.8 | 26.0 | 0.2 |
Petco Park | SDP | 101.5 | 102.1 | 0.6 | 25.9 | 26.1 | 0.2 |
T-Mobile Park | SEA | 102.2 | 101.5 | -0.7 | 25.7 | 25.8 | 0.1 |
Oracle Park | SFG | 101.5 | 102.8 | 1.3 | 26.0 | 26.7 | 0.7 |
Busch Stadium | STL | 101.7 | 101.1 | -0.6 | 25.7 | 25.9 | 0.3 |
Tropicana Field | TBR | 101.5 | 101.9 | 0.4 | 26.5 | 26.2 | -0.3 |
Nationals Park | WSH | 101.2 | 102.5 | 1.3 | 25.4 | 26.0 | 0.6 |
The chart is sortable, so you are able to examine any changes in fly ball quality within this broader 95–109 mph exit velocity, less-than-30 degree launch angle group as you wish. I did attempt to examine the impacts, if any, of these exit velocity changes on home run rates, and there was a moderate correlation (r-squared of 0.28) that is important to keep in mind. (The r-squared between launch angle difference and home run rates was just 0.01.) Even still, after using a model to fit the change in home run rates of each ballpark to account for any exit velocity changes, there was quite a significant difference for certain parks that is more likely to be the result of non-player effects, like the baseball or the addition of a humidor.
First, here are the raw home run rates for these types of fly balls for each ballpark from 2019 to ’21. I ran a difference in proportions test to compare the rates from both years; parks highlighted in red experienced a significant change at the alpha = .01 level, and parks highlighted in yellow experienced a significant change at the alpha = .05 level. The table is sorted by raw percentage difference, but the statistical tests incorporate sample size, which is why certain rows may have smaller differences but are still significant at a more extreme alpha level:
Park | Team | 2019 HR% | 2021 HR% | Difference | P-Value |
---|---|---|---|---|---|
Oakland Coliseum | OAK | 67.5% | 35.8% | -0.317 | 0.000 |
Busch Stadium | STL | 50.0% | 22.9% | -0.271 | 0.000 |
American Family Field | MIL | 66.7% | 39.6% | -0.270 | 0.000 |
Kauffman Stadium | KCR | 55.6% | 30.0% | -0.256 | 0.000 |
Camden Yards | BAL | 75.3% | 50.0% | -0.253 | 0.000 |
Dodger Stadium | LAD | 72.3% | 47.6% | -0.247 | 0.000 |
Citi Field | NYM | 50.0% | 28.9% | -0.211 | 0.015 |
Guaranteed Rate Field | CWS | 63.5% | 43.5% | -0.199 | 0.002 |
Coors Field | COL | 55.7% | 37.1% | -0.186 | 0.003 |
Comerica Park | DET | 49.1% | 31.7% | -0.175 | 0.010 |
T-Mobile Park | SEA | 59.4% | 43.1% | -0.163 | 0.011 |
Minute Maid Park | HOU | 65.5% | 50.0% | -0.155 | 0.006 |
Progressive Field | CLE | 67.8% | 53.3% | -0.145 | 0.055 |
Citizens Bank Park | PHI | 62.3% | 47.9% | -0.144 | 0.057 |
Great American Ballpark | CIN | 70.7% | 57.8% | -0.129 | 0.033 |
Angels Stadium | LAA | 71.7% | 60.0% | -0.117 | 0.093 |
Wrigley Field | CHC | 57.4% | 47.1% | -0.104 | 0.107 |
Yankee Stadium | NYY | 51.1% | 40.7% | -0.104 | 0.165 |
Truist Park | ATL | 63.8% | 55.6% | -0.082 | 0.219 |
Target Field | MIN | 53.7% | 45.8% | -0.079 | 0.181 |
Petco Park | SDP | 58.8% | 55.1% | -0.037 | 0.701 |
Tropicana Field | TBR | 50.9% | 48.9% | -0.020 | 0.897 |
Fenway Park | BOS | 39.5% | 38.1% | -0.014 | 0.924 |
loanDepot Park | MIA | 44.0% | 43.2% | -0.008 | 1.000 |
PNC Park | PIT | 54.3% | 54.2% | -0.002 | 1.000 |
Chase Field | ARI | 42.3% | 44.9% | 0.026 | 0.824 |
Nationals Park | WSH | 44.8% | 53.8% | 0.090 | 0.243 |
Oracle Park | SFG | 37.0% | 52.5% | 0.155 | 0.061 |
What’s interesting to note here is that 25 of the 28 parks listed experienced some level of decrease in home run rate, which aligns with the broader trend that I broke down about last month. It is also interesting that Oracle Park in San Francisco has seen a 15.5-point bump in home run rate, likely due to the Giants moving their fences in for the 2020 season. If we’re comparing 2019 to ’21 data, that change would explain the large positive difference in home run rate there.
On the other end of the spectrum, look at Oakland, which saw a drop of almost 32 points in home run rate on these “wall-scrapers.” If you plot these types of fly balls in a spray chart, the difference is noticeable to the naked eye. In 2021, there are more fly balls in play compared to 2019, many of which were ultimately caught for an out (indicated in magenta):
Similar trends can be observed for other ballparks as well. Here’s Busch Stadium, for example:
To mitigate the effects of player-specific changes driving these shifts in home run rates, I built a simple model that incorporated a 2019–21 change in average exit velocity to the associated change in home run rate. Without getting too deep into the mathematical details, some parks — like Oakland Coliseum and Busch Stadium — still had significantly large residuals, meaning that even after accounting for any exit velocity changes, the difference in home run rate was notable. Exit velocity is not the only way to mitigate player-specific effects, but it is an important factor to keep in mind when thinking about park-specific home run rates.
Ultimately, the answer to why certain parks might have been more impacted by the new run environment than others is difficult to unpack. As mentioned, we don’t know for sure which parks added the humidor. Additionally, the interaction between outfield space and outfield wall height may also play a role, as could climate and, more specifically, weather so far this season. And, as always, there’s definitely randomness in here as well. But perhaps more than understanding why we’re seeing such significant changes at the ballpark level, it’s first important to acknowledge that some huge differences exist in how the baseball (and potential humidor) is affecting how the ball flies in each environment.
Devan Fink is a Contributor at FanGraphs. You can follow him on Twitter @DevanFink.
This is a fascinating thing but I do think you would need to take into account the “getting better pitchers” / “getting worse hitters” aspect of this in a more direct way. The Orioles got way, way better pitchers; the Mets thumpers have all been injured; the Brewers both lost all their thumpers and their pitching got way better. But there are a few stadiums that are harder to explain. These are St. Louis, Seattle, and Cincinnati. What’s going on there is straight-up weird.
Absolutely agree. Hope that these numbers can be the jumping off point for a more robust analysis by someone else!
Could there also be a weather issue of comparing a full season to a partial one? Sorry if I missed it, but is the 2019 data just the first two months as well?
It is just games played on or before May 31 for both 2019 and 2021, yes.
I agree that personnel changes are a tough confounder for park analysis to overcome. Though as a frequent viewer of Brewers games, I can attest that I’ve seen far more balls that appear to be hit “on the screws” (or thereabouts) go nowhere this year. Visual contact quality that chased outfielders back toward the warning track the last few years is now resulting in outs caught by stationary fielders. Even the crack of the bat oft proves misleading. As my eyes and ears continue to fool, it’s hard not to presume more external forces are at play.
All this is to say I would be pretty shocked if AmFam/Miller is not one of the stadiums using a humidor.
I feel this way about Dodger Stadium, too (and the data! wow!), but in my most paranoid state, I cannot imagine anyone thinking, “Let’s secretly install humidor *in Los Angeles*”
Not counting openers and ranked by WAR, the White Sox rotation in 2019 (starts):
Lucas Giolito (29)
Reynaldo Lopez (33)
Ivan Nova (34)
Carlos Rodon (7)
Dylan Cease (14)
Hector Santiago (2)
Odrisamer Despaigne (3)
Dylan Covey (12)
Ross Detwiler (12)
Ervin Santana (3)
Manny Banuelos (8)
And in 2021:
Carlos Rodon (9)
Lance Lynn (9)
Dylan Cease (11)
Lucas Giolito (11)
Michael Kopech (3)
Dallas Keuchel (11)
Jimmy Lambert (1)
Which group would you expect to give up more homers?
Better pitchers/worse hitters might be a factor, but what is the coefficient next to that variable? A statistically significant change in pitcher and hitter quality in just 2 years seems unlikely.
The ball seems to be the story here. Oakland with a 32% decline in HR% on hits with that LA/EV…6 teams with a >25% decline… These are astounding numbers.
Give me a break. Pitchers aren’t any better. This is ALL on the hitters. When you swing for a home run EVERY time, because you stupidly think that “exit Velocity is a stat that matters, you are going to miss a lot of times. Batters of past generations were SMART. They would shorten up their swing with 2 strikes, just trying to make contact. A base hit is better than a strikeout, after all. Or, if they only need a base hit to win, or a sacrifice fly or slow ground ball. And the bottom of the order rarely tried to hit a home run. And if a team was stupid enough to leave one entire half of the infield wide open for you? Thank you very much for the free bunt base hit. Don Mattingly sure as hell wouldn’t se stupid enough to hit into a shift.
Nolan Ryan would legit have 7500 strikeouts, 12 no hitters and AT LEAST one 25 strikeout game if he pitched today and were allowed to stay in as long as he did. Not because he’d be better. ALL because the hitters are idiots. Just another example of how analytics is destroying the sport for many many fans. There is nothing exciting about strikeouts and home runs. Even the home runs are very fleeting excitement, followed by bases empty and more strikeouts. Even pitchers’ duals aren’t exciting anymore because they usually are the result of bad hitting and not great pitching. A team striking out 15 times in a game is not a rare occasion when they ran up against great pitching. It is something that happens to almost every team once a week. and too often in 2-1 games or 1-0 games, it is not pitchers painting the black and keeping the batters’ off balance. It is hitters swinging at balls that bounce 8 inches in front of home plate because they decided before the windup that they were going to swing.