Seattle Seahawks Have the Strongest Home Field Advantage in the League, But Why?

Based on the last five years, no team has had as large a differential in its home/road performance as the Seattle Seahawks, compared to public expectation as measured by point spreads. Point spreads already account for some home advantage. Seattle, for example, averaged being favored by 0.4 points at home, but were a 5.0 point underdog away from 2008 to 2012. That shows a difference of 5.4 points, which once you half it, is similar to the roughly 2.5 to 3 point difference generally attributed to home field advantage in the point spread.

The question, then, is whether this is a market inefficiency when it comes to Seattle, and whether the public is properly accounting for the home field advantage in Seattle. According to Fortenbaugh, Seattle was 4.2 points better than expected at home over those five years, and 2.9 worse than expected on the road. That difference of 7.1 points away from expectation is easily the largest in the league.

This is not a new phenomenon. One way is to look at another independent set of information, in this case, the separate five year period before that measured. The years 2003 to 2007 largely featured a different coach (Holmgren), different players (Shaun Alexander, Darrell Jackson, Walter Jones, etc.), and largely different fortunes. Those teams from 2003 to 2007 made the playoffs every season, while the more recent Seahawks teams had losing records until last year’s breakthrough.

From 2003 to 2007, using the same measure as Fortenbaugh uses, the Seahawks were +2.4 points better than expected at home, and -1.2 points worse than expected on the road, for a difference of +3.6 points. I did not run all of the teams for that prior period but am confident in saying that Seattle also had one of the largest home/road splits for the previous five years as well. A couple of outlying results on the road (who can forget this beatdown of the Eagles on Monday Night) really skew the average, but typically, Seattle failed to cover on the road.

In fact, the road “against the spread” records were identical for the two periods: 15-24-1 for each. At home, the Seahawks were 22-15-3 from 2003-2007, and 26-13-1. Add it all up, and if all you did was know nothing about football but just blindly took the home team in a regular season game involving Seattle over the last decade, you would have won 61.9% of the time.

If you flipped a coin 160 times and got heads 62% of the time, you would conclude the coin was rigged. In this case, home field advantage is not identical for Seattle as it is for other teams. Why is that? As with many things, there is not one simple answer, but multiple factors are playing a role. Here are what I believe to be the largest causes for home field advantage/road disadvantage for teams playing in Seattle.

THE CROWD AND STADIUM STRUCTURE: Does this play a role in the home field advantage? I am sure it does. I am also sure that it is an overstated reason for the effect, and certainly doesn’t explain the counter side of the coin, why Seattle is a poor road team. As the Harvard Sports Collective notes here, the stadium architecture does likely play a noise role versus a place like Gillette Stadium. I am dubious, though, of any belief that something about the fans themselves in Seattle makes the difference; there are passionate fans in many NFL cities.

THE GEOGRAPHIC ISOLATION OF SEATTLE: Chase Stuart wrote about this in December and so I will build off many of those points. I don’t think it is any coincidence that Seattle also happens to be the most geographically isolated location in the NFL. (And on a related point, if the league ever does actually expand to London, my prediction is that team will show the largest home/road splits). The closest NFL location to Seattle to the South is the Bay Area, to the East is Minnesota, and to the Southeast, Denver. In fact, the first thing I ever wrote, when I was just a commenter with a screen name, was about travel distances and home field advantage. Which leads to . . .

SEATTLE IS A UNIQUE CLIMATE FOR THE NFL: Seattle’s home field advantage has actually increased since they moved outdoors to CenturyLink field. When they played in a dome, they were far from the only team to do so, so that while travel distances and time zones were a factor, a unique weather situation was not.

Here is a look at Seattle’s average weather during football season, using data from weather.com. For comparison, I list weather from two other West Coast teams (San Francisco and San Diego) as well as a Midwest (Kansas City), Great Lakes (Green Bay) and Northeast (New York City) location. The first number under each city is average temperature (Fahrenheit) for each month, the second column is average precipitation (in inches).

San Francisco is, of course, the closest in type of weather. However, Seattle is more than ten degrees cooler by November and December. The precipitation pattern is similar in that both experience more in the later months, though Seattle gets more of it. Early in the season, Seattle is one of the coolest locations, right there with Green Bay. By the end of the season, most of the cities in the Midwest and Northeast have surpassed Seattle, but it is still cooler (and rainier) than the moderate climate teams from the South, Southwest, and Atlantic Coast.

Climate differences do have a large impact, and in Seattle’s case, the climate is sufficiently different from just about every other NFL location that it remains true for Seattle regardless of who they play. If there were teams in Vancouver, Portland, and Tacoma, we would not see the same degree of home field advantage. The question going forward is whether the betting public notices, and lines are adjusted.