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June 29, 2008

Lies, Damned Lies

Home-Field Advantages

by Nate Silver

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Home-field advantage is making a little bit of a comeback this year, with the home team thus far having won 56.2 percent of major league baseball games. This is actually down a few ticks from where it was several weeks ago; at the beginning of June, home teams had won almost 58 percent of their games. Nevertheless, this is quite high by the standards of recent history. Prior to World War II (when travel was more burdensome and road trips much longer), home-field advantage was more profound in baseball, but since then it has been exceptionally stable, with the home team winning about 54 percent of games each season. So, is there something systematic that is causing the home-field numbers to increase this year? Or has it just been some kind of statistical fluke?

To address this question, we first need to consider why home-field advantage exists in the first place. In baseball, the home team has a couple of built-in tactical advantages: in the National League, for example, by batting last it can pinch-hit for the pitcher while getting one extra inning out of him. Regardless of league, the home team knows exactly how many runs it needs to play for, which making ninth-inning strategy easier.

However, there are other elements that differ from park to park. Some parks favor hitters, and some favor pitchers. Some parks are asymmetrical-favoring batters that hit from a certain side of the plate-whereas others treat all hitters equally. If teams are becoming more aware of such differences and more aware of how to exploit them, that could explain the uptick in home winning percentage.

Let's examine six or seven variables that might conceivably be related to home-field advantage, rating each major league ballpark from 1 (lowest) to 5 (highest) based on a somewhat subjective assessment of those categories.

Asymmetry

As every Strat-o-Matic player knows, it is usually an advantage to have an asymmetrical ballpark, as you can load up on hitting talent from whichever side of the plate that park tends to favor. As more teams get asymmetrical parks-with "quirky" HOK-designed retroparks replacing 1970s-era cookie cutters-there may be more opportunity to exploit such advantages.

Based on a combination of objective (the lefty/righty splits in my park factor databases) and subjective factors, this is how we might classify the parks from most asymmetrical to least. Some parks behave counterintuitively. For example, U.S. Cellular Field is very nearly symmetrical, but for whatever reason-perhaps the prevailing wind conditions in the area-it has tended to be much friendlier toward left-handed hitters. That said, here are my ratings for how much the parks "lean":

5: Boston, Houston, Pittsburgh, San Francisco
4: Baltimore, Cleveland, Detroit, Florida, Philadelphia, NY Yankees
3: Chicago Cubs, Chicago White Sox, L'Anaheim Angels, Minnesota
2: Arizona, Colorado, Oakland, San Diego, Seattle, Texas
1: Atlanta, Cincinnati, Kansas City, LA Dodgers, Milwaukee, NY Mets, St. Louis, Tampa Bay, Toronto

Hitter/Pitcher Balance

There are a few further strategic options that come into play when you have an imbalanced facility that either to favor either offense or defense. A team in a power-friendly park can concentrate on pitchers with strong groundball ratios, for instance, while one with a big outfield can employ fly ball-prone pitchers that might get taken advantage of elsewhere. Teams in pitchers' parks may also have something of an advantage as far as limiting their pitch counts and making bullpen management easier.

5 (most favors hitters): Colorado
4: Arizona, Boston, Chicago Cubs, Chicago White Sox, Cincinnati, Kansas City, Philadelphia, Texas, Toronto
3 (neutral): Atlanta, Baltimore, Cleveland, Houston, LA Dodgers (yes, really), Milwaukee, Minnesota, NY Yankees, Pittsburgh, Tampa Bay
2: Detroit, Florida, L'Anaheim Angels, NY Mets, Oakland, Seattle, San Francisco, St. Louis
1 (most favors pitchers): San Diego, Washington

Fences and Caroms

The presence of quirky fences or caroms in the outfield is another potential source of advantage. Learning to play the Green Monster or the caroms in Dolphin Stadium can take years to master, providing a potential defensive advantage to the home club.

5: Boston
4: Florida, Houston, Minnesota, San Francisco
3: Baltimore, Chicago Cubs, Cleveland, Pittsburgh
2: Colorado, L'Anaheim Angels, Oakland, Philadelphia, San Diego, Texas
1: Arizona, Atlanta, Chicago White Sox, Kansas City, LA Dodgers, Milwaukee, NY Yankees, NY Mets, Seattle, St. Louis, Tampa Bay, Toronto, Washington

Weather

Somewhat parallel to the idea of adjusting to peculiar outfield dimensions is adjusting to peculiar weather conditions-be they rain or snow or wind or gnats or seagulls or any other kind of pestilence. Listed from wildest weather to mildest, and accounting for the presence of domes:

5: Colorado (altitude), San Francisco (wind/cold), Texas (heat)
4: Chicago Cubs (wind), Florida (frequent rain delays, rainouts)
3: Atlanta (heat)
2: Arizona, Boston, Chicago Cubs, Cincinnati, Cleveland, Detroit, Houston, L'Anaheim Angels, Oakland, Seattle, St. Louis
1: Baltimore, Kansas City, LA Dodgers, Milwaukee, Minnesota, NY Yankees, NY Mets, Philadelphia, Pittsburgh, San Diego, Tampa Bay, Texas, Toronto

Domes

Speaking of which, there may be advantages to playing in a dome too. Domed stadiums can be associated with advantages in outfield defense, as most outfielders won't be used to tracking balls on a canvas- or concrete-colored background. Teams with retractable roofs may have another advantage still, as they can leave the dome open or closed depending on the particular whim of the pitcher, as the Diamondbacks used to try and do with Curt Schilling.

Yes: Arizona (retractable), Houston (retractable), Milwaukee (retractable), Minnesota, Seattle (retractable), Tampa Bay, Toronto (retractable)
No: Everybody Else

Attendance and Atmosphere

If home-field advantage is mostly a psychological phenomenon, then perhaps it is larger for teams that enjoy consistently larger attendance, especially if their ballpark is designed such that fans can have more interaction with the action on the field. Let's group the teams by some rough average of attendance and fan enthusiasm over the past couple seasons.

5: Boston, Chicago Cubs, LA Dodgers, NY Yankees
4: Detroit, L'Anaheim Angels, NY Mets, Philadelphia, Seattle, San Francisco, St. Louis
3: Baltimore, Chicago White Sox, Colorado, Houston, Milwaukee
2: Arizona, Atlanta, Cleveland, Minnesota, San Diego, Toronto
1: Cincinnati, Florida, Kansas City, Pittsburgh, Tampa Bay, Texas, Washington

Travel Distance

Finally, it's a much longer slog up to Seattle or down to Miami than it is to most other places in the country. I have actually found some relationship between home-field advantage and travel distance before. As I pointed out in It Ain't Over, for instance, those teams furthest from baseball's center of gravity during World War II (which would roughly have been Pittsburgh in those days) tended to have larger home-field advantages, as war-time travel restrictions made train travel much more crowded and much less luxurious than it usually was. Teams in then far-flung cities like St. Louis and Boston tended to have larger home-field advantages.

5: Florida, San Diego, Seattle, Tampa Bay
4: L'Anaheim Angels, LA Dodgers, Oakland, San Francisco, Toronto
3: Arizona, Boston, Houston, Texas
2: Baltimore, Colorado, Detroit, Milwaukee, Minnesota, NY Yankees, NY Mets, Washington
1: Chicago Cubs, Chicago White Sox, Cincinnati, Cleveland, Kansas City, , Pittsburgh, St. Louis

---

Going back to the larger problem, which teams have actually had the largest home-field advantage of late? This can be calculated simply by subtracting the team's winning percentage on the road from their winning percentage at home in all games since 2004. For the Cardinals, I use only their numbers since they moved into their new ballpark in 2006, and I exclude the Nationals entirely since they've played only half a season in their new facility.

Table 1: Home and Road Winning Percentages, 2004-2008


Team   HomeW%  RoadW%  Difference
TBA     .516    .340     .177
MIL     .569    .394     .175
BOS     .655    .508     .147
COL     .530    .385     .146
TOR     .558    .419     .139
PIT     .500    .371     .129
SLN     .572    .450     .122
HOU     .572    .455     .117
SEA     .507    .391     .116
TEX     .556    .443     .113
NYA     .645    .537     .107
ATL     .585    .481     .104
MIN     .586    .494     .092
CIN     .508    .418     .090
LAN     .554    .466     .088
KCA     .428    .346     .082
OAK     .575    .501     .073
CHN     .549    .479     .070
CHA     .568    .499     .070
CLE     .558    .496     .062
ARI     .490    .429     .062
NYN     .548    .492     .057
FLO     .515    .463     .052
SDN     .541    .496     .045
DET     .523    .479     .044
SFN     .496    .460     .036
LAA     .591    .559     .031
BAL     .478    .448     .029
PHI     .544    .530     .014

The kings of home-field advantage over the past several seasons have been the Rays and the Brewers, each of which have about an 18-point gap between their home and road winning percentage. Whether this is a result of their playing particularly well at home or particularly poorly on the road, I don't know; if you're looking for the team that has actually won the most home games, that would be the Red Sox. It's also funny to see the Phillies at the bottom of the list when half the time their fans seem to be rooting against them.

Do we find any relationships between these numbers and the variables that we described before?It turns out that only one of them has a statistically significant relationship with the home/road win percentage gap, and that is playing in a dome. Arizona, Houston, Milwaukee, Minnesota, Seattle, Tampa Bay, and Toronto have combined for a 54.3 percent winning percentage at home since 2004 versus 41.7 percent on the road, a difference of 12.6 points. By contrast, the other (non-dome) teams have had a gap of 7.7 points between their home and road performances.

The question I don't yet have an answer for is the 'why'. Could it be, as I hypothesized before, that playing in a dome may provide some advantages in terms of outfield defense? Might it be that playing baseball under a roof is a miserable experience that demeans the human condition and tends to depress the visiting club?

For the time being, I'll leave you with the stathead's version of the dog-ate-my-homework excuse: this is a direction for further research which I'll revisit again in a couple of weeks. It does not appear, however, that the uptick in home-field advantage this year has anything to do with teams taking better strategic advantage of their home facilities, as both teams that play in bland ballparks like Atlanta and idiosyncratic ones like Boston have had quite strong home/road splits thus far this year.

Nate Silver is an author of Baseball Prospectus. 
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