Home field advantage is real, but it’s not because of the 12th man

October 18, 2012

The Redskins ended their eight-game home losing streak Sunday with a convincing win over the Vikings. Home teams usually enjoy a substantial advantage, and a lot of hay is made with claims that some stadiums and crowds lend their teams greater home field advantage than others. But the effect is widely misunderstood, and the causes of home field advantage are a bit mysterious.

First, let’s look at some basics about home advantage in the NFL. Home teams win about 57% of the time, a rate that has been consistent through eras. It’s not a guarantee that every team will win more at home than on the road, but a long term statistical average. Home teams outscore their guests by just more than 2.5 points. You might see self-proclaimed experts dicing up the situations into ‘splits,’–such as home teams defeating visitors by 6.1 points under the lights on turf. You can safely ignore the vast majority of these kinds of analysis, because they are based on tortured data and very small samples.

Keep in mind that a lot of what happens in any sport is random. If we flipped a penny, we wouldn’t expect to see exactly eight heads and eight tails on every run of 16 flips, even though we know the chances are 50/50. On some rare occasions we’d be likely to see very few heads or vice versa. No explanation is needed when we see unlikely strings of outcomes. In any one year, or in a set of years, some teams are bound to appear to have stronger home field advantages than others, but this is mostly a statistical illusion.

I say “mostly an illusion” because some circumstances do enhance home field advantages. When two opponents are roughly similar in ability, home advantage is enhanced in terms of the probability of winning. When teams are likely to score and allow similar point totals, those 2.5 points become more critical to the outcome. Home advantage for NFL teams with similar statistics result in a home winning rate of well over 60%.

This is partly why some teams appear to have better home crowds. Seattle, for example, is particularly proud of its 12th man. Its stadium is commonly accepted as one of the NFL’s loudest and Seattle has enjoyed one of the strongest home-away splits in sports. But this should be expected, 12th man or not, because the Seahawks have consistently been one of the most mediocre teams in the league. Because there are bound to be far more average teams than elite or very poor teams, Seattle has faced many more teams of equal ability over recent seasons.

Home advantage is universal in sports. Whether it’s a team or individual sport, professional or amateur, virtually all athletic competitions have a home advantage. Fans like to believe they are the cause. Communication is essential in football, audibles and snap counts being the most obvious. The players themselves tell us it makes a difference. As plausible as this explanation may seem, the evidence suggests home advantage does not come from the crowd.

Home advantage has been shown to reliably exist in competitions without any crowd. And although some studies suggest that the crowd may influence and bias game officials, this can be explained by the fact that better-performing teams have less need or incentive to commit a foul or penalty. A cornerback in proper position has little need to commit pass interference. Research studies on other causes, such as travel distance or sleep duration, have failed to provide strong evidence of their effects.

The strongest theory of the cause of home advantage is environmental familiarity. It’s not just the nooks and crannies of an unusual baseball park or a giant video screen hovering above the field. It’s everything. It’s the slight anxiety and uncertainty everyone feels in an unfamiliar place. The strongest clue that environmental familiarity is at work is that as players become more familiar with their environment, home advantage decreases. Division opponents, who play twice per season, show a lower home win percentage than other opponents. And NFL home advantage decreases as a game progresses. Home teams outscore visitors most in the first quarter and less in each subsequent quarter until home advantage disappears in overtime.

Studies of hockey and soccer players at home showed heightened pre-game levels of testosterone, the hormone most associated with aggression. These findings corroborate a line of research that shows heightened territorial behavior in a wide range of competition. In a way, we’re not too different from our fellow mammals that continuously mark and guard their territory.

As fans, we like to feel we’re part of the team and that we’re contributing to the win. Sadly, that’s mostly a romantic delusion. Home advantage would be there whether we’re screaming from section 101 or watching from our couches.

Brian Burke is the creator of Advanced NFL Stats, a Web site about football, statistics and game theory.

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Mike Jones · October 18, 2012

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