Wednesday, May 29, 2013

Home Field Advantage: Distance Doesn't Matter, But Time Zones Do

The existence of home-field advantage is well-known and not in dispute, but the magnitude of the home team's advantage is not often discussed. Additionally, it's reasonable to assume that while some of this effect is due to crowd noise or unconscious referee bias there may also be a component that results from the distance the away team needs to travel. It turns out that while the absolute distance traveled doesn't affect home-field advantage, teams that have to cross time zones — specifically traveling East — do almost 10% worse than on average.
Whenever there's a big game, TV commentators always like to spend some of their pregame chitchat on the topic of home field advantage. When the visiting team has a false start or delay of game, the announcers are quick to blame crowd noise. Even after a team has locked up a playoff spot they still play hard until they've secured (or have no chance at) home-field for the playoffs. The subject comes up enough that it's been the subject of at least one hour-long NFL network production. One of the reasons the Super Bowl is held at a neutral location is to remove this advantage (and it's also why basebal and the basketball organize their playoff series the way they do).

But what exactly is home field advantage? Is it some constant that applies for all visiting teams? Is it only applicable for those teams who play in loud stadiums (Seattle, Kansas City) or locations with hostile climates (Green Bay, Buffalo)? How strong is the effect?

Anyone who remembers the last several Super Bowls has good reason to be skeptical of the power of playing at home: 5 out of the last 8 Super Bowls have been won by the 4th seed or worse. In the same time period, the number one seed — the team with home field advantage — from either conference has only won once.

Obviously just looking at Super Bowl winners doesn't provide a very large sample size, and there can be other complicating factors — for instance in 2010 the Jets secured a wild-card spot with an 11-5 record, but had to go to Indianapolis because the 10-6 Colts won their division. So for this experiment we'll look at nearly two decades of regular season games, and probe the role that distance plays in the effect.

I downloaded game information (teams and scores) for all regular season games between the 1995 and 2011 seasons from using a Python web scraping script. Going further back in time might be useful, but it runs the risk of biasing your data if teams change the way they travel.

Determining the home time zone for each team is generally straightforward, except for the Cardinals. Since most of Arizona doesn't observe daylight savings time, during the summer months and part of fall the Cardinals are in the pacific time zone. But in the winter, when the rest of the country falls back, Arizona goes to Mountain time. For simplicity's sake I used Week 8 of the regular season as a cutoff for determining which time zone the Cardinals were in, as that's usually around when the switch happens.

The first question to answer is whether home field advantage exists at all. Counting up the entire data set, the home team wins 58.0% of the time, with a 1-sigma bootstrapped error of 0.76%. That's pretty significant — If a team played all 16 of their games at home on average they'd win one extra game per season!

So home field advantage is clearly a real effect. But what controls the advantage? This result is league-wide and over many years, so we can discount stadium-specific effects.

A natural next step is to investigate the effect distance has on home-field advantage. Toward this end I computed the stadium-to-stadium distance between the two teams for every game. A plot of the win percentage of the home team as a function of the distance the away team had to go is shown in Figure 1. The error bars are 1-sigma bootstrapping errors, and the red bar shows the average value of the whole dataset.
Figure 1
Figure 1: Home team win % as a function of how far the away team had to travel. The red line shows the average home-field advantage.

The widths of the bins were chosen to keep the number of samples roughly constant in each bin, which is why the bins at small distances are narrower than the bins at large distances. While there is a general trend towards a larger advantage when the away team has to travel a long distance, it's very weak. Only the smallest-distance bin is inconsistent with the global average, and it's not off by very much.

Pure distance isn't the only deleterious effect of traveling, however. Crossing time zones can mess with circadian rhythms, and an East coast team has three time zones to travel to play one of the California or Washington teams.

Figure 2 shows the relationship between the number of time zones traveled and the home team's win percentage. Firstly, when the home and away teams are both in the same timezone, the home-field advantage is lower than the average — when the Dolphins travel up to Buffalo the Bills don't have a full 58% home field advantage.
Figure 2
Figure 2: Win % as a function of the number of time zones crossed by the away team. Lines are the same as in Figure 1.

From East to West the time zones get increasingly negative — New York's time zone is -5 hours, while San Francisco's time zone is -8 — so an East coast team traveling to a West coast team would be on the left side of the plot.

When a team travels one or more time zones West, the effect of home-field advantage is roughly the same as the overall average of 58%. However, when teams go East the benefits to the home team are well above the average, hovering closer to 65% and significantly larger than their errors.

Discussion and Conclusions
It seems pretty clear that while home field advantage is very real, it's more dependent on time zones than pure distance. What's really interesting is that the home team only gains an advantage when the visitors are traveling East; there is essentially no additional benefit (over the average home field advantage) to hosting a team which has come to the Pacific Coast from somewhere else in the country.

This finding has consequences for NFL scheduling. The league goes out of its way to ensure that when teams from one division play another, each team plays the same number of home and away games. This consideration is admirable, however it falls somewhat short of ideal given these results, because both the AFC and NFC West have a team in the central time zone (Kansas City and St. Louis). So when an East Coast team hosts the Chiefs instead of another AFC West franchise, they lose out on the added home field advantage.

This discovery has serious ramifications for deciding where to place franchises as well. An NFL owner's job is to do everything in their power to win a Super Bowl. If you know that placing a team on the West Coast will put you at a disadvantage, isn't it your duty as an owner to make sure your team plays in the East? This is especially relevant given the current attempts to put a team in Los Angeles: any owner looking to buy into a new L.A. team is automatically going to be at a disadvantage.

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