The Other Coors Effect

I have a slight problem.

I’m writing today about the Coors Effect and whether a similar effect, or a reverse effect, occurs at other ballparks. The problem is that there are two kinds of Coors Effects, and the one that is better known is not the one I am writing about. This means, before I can discuss what I am writing about, I briefly need to talk about what I’m not writing about, if only to say, “That’s interesting, but it’s not what I mean.”

The original definition of the Coors Effect is the great increase in offense and scoring, relative to other major-league venues, that happens at Coors Field in Denver. The effect happens less because of the field than because of the city, or specifically the altitude of the city. Lower air pressure at the mile-high altitude of Denver means less air resistance on batted balls, so they fly farther. It also means breaking pitches have less bite, so they move less and are more hittable. (It also means pitches arrive at the plate a little faster, but this defense-oriented effect is swamped by the offense-oriented ones.)

Even casual fans are aware of this effect, though that doesn’t mean we fully comprehend it or adjust for it. We know enough to see through part of a Rockies batters’ gaudy offensive numbers but perhaps don’t know how much to discount them. This has, for example, made the Hall of Fame candidacies of Larry Walker and Todd Helton more contentious than they would have been otherwise. People are split over their true value more than usual, and the Coors Effect is why.

That’s interesting, but it’s not what I mean.

My version of the Coors Effect springs from observations originally made by Eno Sarris in 2015. He found that Rockies hitters had the widest home-road splits in the majors over multiple years, even accounting for park effects. Sarris postulated, both from the stats and from player interviews, that Rockies hitters were pitched differently at home, and this made it harder for them to adjust to the more standard approaches of the pitchers they would see on the road.

Other writers have followed Sarris’ lead in examining this second Coors Effect. Mike Petriello at MLB.com, for example, looked at several players with substantial careers both in Colorado and elsewhere, and found the ex-Rockies’ performance on the road rebounding once Coors wasn’t their home field. He also found a team-wide gap in strikeout rates,—18.5 percent at home, 23.9 percent away—in 2015.

The second Coors Effect is real, though the reasons for it aren’t as easily pinned down. Baseball has a funny way of sometimes wriggling loose from cause-and-effect relationships that sound eminently plausible. Fortunately, I don’t need the reasons pinned down. What I’m seeking is something simpler, something that might even shed light on that cause-and-effect question.

I want to see whether something like the second Coors Effect, or opposite of that effect, happens at other ballparks. I want to know if other stadiums favor their home offenses more than we’d expect, or disfavor them so that the batters almost can’t wait for road trips.

No Place Like Home?

My study encompasses 23 years of home-road stats, which doesn’t mean I’m always getting 23-year sample sizes. Teams move into new ballparks (occasionally in new cities); old ballparks get reconfigured such that batting conditions change; humidors change the properties of baseballs in play. (I definitely will have things to say about that.) This breaks up the samples for many teams into two or more pieces. I will carefully keep this in mind, mine and yours, throughout this piece.

For every team season, I took the team’s offensive sOPS+ for home and away games. sOPS+ measures performance relative to the league, on the familiar scale of 100 being average. I likewise took the single-year park factors for each team season, using those to adjust the difference between home and road sOPS+ factors. I could not use multi-year park factors, which are often more reliable, because this would frequently combine different ballpark configurations with each other. What resulted was a series of offensive home-road differences adjusted to park and league, with zero as the average** and positives meaning better performance at home. (The average actually ended up around -0.45 for a single team season. Since the team scores are all relative to the league, meaning to each other, I did not further adjust them.)

The team with the biggest home advantage—or the biggest road disadvantage, if you like—was indeed the Colorado Rockies. This comes with two conditions, though. First, the margin between the Rockies and the second-place team is not nearly as wide as one might have expected. Second, the Coors Effect at Coors began in earnest only in 2002. In the six years before, the effect had been positive but not greatly so. This changed when the humidor came to Coors Field. In the 17 full seasons since, the annual home advantage for batters jumped from +0.9 to +5.0, meaning Rockies batters hit five points better than expected on the OPS scale at home than away, adjusted for park factors and the general tendency of teams to play better at home.

This leads to a surprising suggestion. It might not be the altitude, but the humidor, that produces this particular Coors Effect. If that is the case, we can expect a similar effect to happen when the humidor is introduced at another ballpark. That it was, just last season at Chase Field, home of the Arizona Diamondbacks. They hoped to combat a similar offense-friendly combination of climate (especially the dry air) and altitude (second-highest in the majors, though not nearly as high as Denver).

In the first 20 seasons of Chase Field, the D-backs had an adjusted home batting advantage of -0.5 points of OPS+, meaning a slight disadvantage. In 2018, with the humidor, that number shifted substantially, to -5.5. Instead of favoring the Snakes’ batters, the change got them longing for the road.

Some readers will promptly question whether one season is long enough to get a solid statistical handle on the effects of the humidor. From the data I’ve drawn from all 30 teams, I can answer that their qualms are justified.

I calculated standard deviations for every park configuration lasting at least two years, totaling 56 for the 30 teams from 1996 to 2018. This includes the two-year stretch when the Montreal Expos split their home dates between Stade Olympique and Estadio Hiram Bithorn in San Juan, Puerto Rico. The great majority of them, 46 out of 56, had standard deviations between five and 10, meaning roughly two-thirds of the values would fall within that distance of the mean. As a pertinent example, the standard deviation at Chase Field in the 20 years before the humidor was 7.75. A deviation of five points from the established average would therefore be nothing special, or even that noteworthy, at Arizona’s ballpark.

This wide variation in home offense numbers carries over to our original case. For the 17-year humidor era at Coors Field, the standard deviation of the home-road difference for Rockies batters is 8.66. The difference was as high as +23.4 points in 2014 and as low as -10.9 points in 2011.

This is not to say the Coors Effect is mythical, an artifact of small samples and high volatility. I think it is well enough established that some effect favoring Rockies’ home offense over the road is happening, though it might not be as large as supposed. It is to say the opposite single-season effect of the humidor in Arizona has not demonstrated anything yet. It probably will not demonstrate anything, even if it hovers around the same size, until several years have passed. (And by the usual range of standard deviations, we shouldn’t expect it to hang steadily around the same size.)

This also means a big recent change in Atlanta is also well short of proven. For the 20 years the Atlanta Braves called Turner Field home, Braves batters had an average -3.1 score at home, a low number but not an outlier. For the Braves’ first two seasons at SunTrust Park, the scores were -5.5 and -20.7. The second-year figure is a shocker, lower than anything Turner Field ever produced, but it’s still one data point. We would need to see a couple more years–at least–of suppressed Braves home batting before making conclusions.

So is there another ballpark tilting the playing field toward its home batters the way Coors Field does? I could produce a table with the exact figures, but it would be complicated by needing to list timeframes and give multiple values for each team due to venue changes and reconfigurations. I’ll save that for a sortable appendix and give a short answer now. Coors Field is close to singular, and for long samples, it stands alone.

In the American League, only four ballpark configurations have produced values even half as high as Coors’ for meaningful spans. Baltimore’s Camden Yards has a +2.5 from 1996 to 2018; Detroit’s Comerica Park has a +2.7 for the 2000-18 period; Minnesota’s Target Field is +3.4 from 2010 to 18. Minute Maid Park in Houston ran a +4.0 from 2000 to 2016 before they razed Tal’s Hill in center field. In the two years since, they’ve registered a -9.5. As I said with Atlanta, that’s not proof, but it certainly draws the eye and merits some watching in future years. The Tal’s-era Minute Maid was the only AL park that got close to Coors’ effect.

In the senior circuit, there are two places that have beaten Coors Field’s mark, but not for very long. First, from 2016 to ’18, Marlins Park posted a +5.3, edging out Denver. This contrasts with a -3.3 in the stadium’s first four years, making the average of the two quite close to zero. I’ll observe in passing that two stadiums with something close to a Coors Effect on home batters are in Houston and Miami, both low, humid, and hot, while Denver is high, dry, and cool.

The other place is at least not humid, and also has a more reliable sample. PETCO Park in San Diego posted an average +14.8 for the 2013-14 seasons, then a +7.6 from 2015 to 2018. This contrasts with a -2.9 from 2004 to 2012, in PETCO’s first configuration. Oddly, that might be the outlier in San Diego, as from 1996 to 2003 Jack Murphy/Qualcomm Stadium posted a +4.4 (through two configurations). It’s not a Coors-sized sample from 2013 on, but six years is getting meaningful, and with an average value twice that at Coors Field.

There have been other NL parks that have made a run at Coors’ number but haven’t quite gotten there. Veterans Stadium in Philadelphia had a +2.8 value from 1996 to its closing in 2003. PNC Park in Pittsburgh has produced a +2.6 from its 2001 opening onward. Montreal’s Stade Olympique showed a +4.1 from 1996 to 2002, the last year the Expos played their home games exclusively there. There is one more configuration favoring the home batters, but I’ll talk about it in better context later.

There is thus perhaps one park matching Coors Field’s effect. Is there one reversing it, making it rough on home batters, or perhaps easier on those batters when they take to the road?

In the American League, not quite. Just three park configurations produce a negative effect as much as half the size of Coors’ positive effect. Progressive Field in Cleveland has held the home team’s batters to a -2.6 from 1996 to the present. Seattle’s Safeco Field (when it still had that name) showed a -3.4 effect from 1999 to 2012; a reconfiguration has left it essentially neutral since then. Angel Stadium notched a -3.1 in the 15 seasons from 1999 to 2013, though it had run a -10.8 for three years in its previous configuration, and in two configurations since has been mildly positive.

Over in the National League, I’ve already covered Atlanta and the original setups at Marlins Park and PETCO Park. Two other old stadiums get a mention. Riverfront Park in Cincinnati sustained a -4.9 from 1996 to 2002, virtually a reverse Coors except for less than half the time. Milwaukee’s County Stadium was even harsher at -6.9, but for only five years.

Among current parks, Washington has seen a -2.5 during its 11 years at Nationals Park (after three seasons of a -3.8 at RFK Stadium). The Cardinals have endured a -3.7 for 13 campaigns at new Busch Stadium. Citizens Bank Park has been a -4.2 in 15 years for the Phillies, a wide swing from their +2.8 at the old Vet. That is definitely close to a mirror-image Coors Effect.

There is a bigger one, though. It has been larger than Coors across a decade and follows a predecessor park that showed similar numbers for longer than that. The unfortunate team in question (for its batters, at least) is the New York Mets, now playing at Citi Field and previously at Shea Stadium.

From 1996 to 2008, Shea Stadium played at a -4.6 for Mets batters. In its first three years, Citi Field showed a reversal at +4.1. Then the Mets powers that be brought in the fences to make things easier for their power hitters. The result was three years of a -12.0 factor. The fences came in again before the 2015 season, and the next four years tallied up a -10.8 factor. The decade of Citi Field, all three configurations taken together, has been a -6.7, a worse minus for home batters than Coors Field has been a plus.

Both Shea and Citi have reputations as pitcher’s parks, backed by the park factor data. I observe with ironic amusement that both times Citi Field’s fences were moved in to raise offense, the field’s park factor inched downward, from 97 to 94.7 to 94.5, measured by Baseball-Reference, for batters. For pitchers the numbers are almost identical, dropping with both re-designs of Citi Field. Mets fans painfully cherish a feeling they are snake-bit, especially where ownership and the front office are concerned. This can only fortify that feeling, probably unhealthily, but the facts don’t care about the fans. (“Neither does the front office,” chorus the Mets fans. Settle down, guys.)

These results can rationally be doubted. Seven years may not be enough time for this stat to stabilize, especially since the stat has a notably wide standard deviation. In this instance, though, SD lends reliability to the results.

The standard deviation for the second setup at Citi Field is a tiny 2.30 and for the third arrangement is 2.51. Those are the two lowest standard deviations for any configurations in the survey; only one other falls below 5.0 (Seattle, 2013-18). Combine the two as though they were identical, and the SD stays steady at 2.41. This trick does not work if you include the original Citi Field setup, ballooning to 8.08. (Original Citi in isolation is 5.89.) The two late versions hold close to an almost identical center, far from that of the original Citi Field.

Citi Field has been, at least since 2012, strongly and very consistently adverse to home-team batters, much more so than Shea Stadium, still quite negative, was. While this tilt against home bats hasn’t lasted as long as Coors Field’s tilt in their favor, the narrow range of figures at recent Citi Field, between -8.3 and -14.3, strongly hints it will endure. Perhaps it will retreat to a lower magnitude, but that leaves ample room for it to remain stronger than the original Coors Effect.

The similarity of the effects at Shea and (current) Citi tempt one to seek a common cause. Citi Field did seem to be designed to resemble Shea’s fence distances, though a bit more asymmetrically for the proper modern aesthetics. There could be something in the particular shape, or the general pitcher-friendliness. I even wonder about the airplanes from LaGuardi, except I don’t think they were re-routed from 2009 to 2011.

It is probably not a geographical effect. The New York Yankees aren’t nearly as extreme, though they are negative: -1.9 for 13 years at old Yankee Stadium, -1.5 for a decade at the new one. Parenthetically, it’s interesting that a team noted for being able to craft its lineup around park tendencies, mainly the short right-field fences at both Yankee Stadiums, didn’t reap home-field batting advantages for it.

If you want to argue for geography as destiny, look west. The Giants and A’s are the only two teams to have a perfectly neutral 0.0 at their current ballparks, and that is with 19 and 23 years of data, respectively. That result is odd because both fields have notable idiosyncrasies, the yawning foul ground of the Oakland Coliseum and the dimensions of Oracle Park that trade homers for triples. It definitely could be coincidence, but it’s quite an exact coincidence.

Having a greater influence on home batting advantage could be domes and turf. Domes seem to moderate the effect. Of the seven domes in the majors, just two have a current effect with a magnitude of 2.0 or greater, positive or negative. The ratio for all distinct configurations in the study range is 33 out of 63. The two fields with artificial grass, Tropicana Field and Rogers Centre, have factors of +0.1 and +0.5, very quiet scores though not enough of them to prove anything.

Summation

The effect of Coors Field on home-team batters is not as singular as we may have thought. PETCO Park has had roughly double the positive effect for half a dozen years, though the sample size is still a trifle small, and the previous configuration there had the contrary effect. In the opposite direction, Citi Field has had a massively negative effect on Mets batters for the last seven seasons, though both of PETCO’s caveats also apply here.

There is a continuum of home-field influences, and while Coors Field is at a far end of it, it’s not really beyond the bounds set by the other 29 teams. If anything, current Citi Field is the true outlier, with PETCO and perhaps SunTrust Park showing potential to be likewise.

The idea that Coors Field has a unique effect on home-team batters, due to altitude or the humidor or both in combination, is not as solid as it appeared. Greater study would be helpful but may not be practical. Large sample sizes of more than a decade in a particular ballpark configuration would be ideal, but teams will not refrain from fiddling with the fences to help satisfy our intellectual curiosity. We’ll have to take the bits and pieces we get.

References and Resources

• Eno Sarris, “‘Coors Field effect’ not limited to games in Denver,” ESPN.com, July 9, 2015
• Mike Petriello, “‘Coors Field effect’ shouldn’t scare off teams interested in CarGo,” MLB.com, December 16, 2015
• The Bill James Handbook, 2010-2019
• Baseball-Reference
• Clem’s Baseball

Appendix