As usual, Aaron Schatz provided some interesting information in his weekly DVOA recap. He was looking into Seattle’s home/road splits, and found that the data support what you already know:

[W]hen you look closer at home-field advantage over a period of several years, almost every team generally has the same home-field advantage, which in DVOA works out to about 8.5% on offense and 8.5% on defense. Teams will see their home-field advantage bounce up and down if you only look at things in eight-game periods that coincide with specific seasons, but if you put together six or seven years of data you are going to end up close to 8.5% difference most of the time. The biggest exception seems to be the four NFC West teams, which over the last decade have enjoyed the four largest home-field advantages in the league. And of those four teams, the biggest exception by far is Seattle.

I don’t doubt that Seattle is a much better team at home than on the road. But here’s the question on my mind today: is Seattle much better at home because, well, they’re much better at home…. or because they simply get more favorable home games than the average team? That might sound like the same thing, but Jason Lisk has done a bunch of research on home field advantage as it relates to climate and distance between the teams.

The table below shows the distance each team has traveled this season. The “road” column represents how many miles the team has traveled when they were the road team while the “home” column shows how many miles their opponents had to travel. Note that I excluded the Patriots/Rams game in London, but instead pro-rated their half-seasons to eight games.

Seattle is the most isolated team in the NFL. Now if an expansion team was place in Vancouver or Portland, my guess is that such a team would fare no worse against Seattle than the Giants do against the Eagles or the Jets against the Patriots. But right now, no one is all that close to the Seahawks:

There are also climate issues at play here. Think of the coldest NFL cities — Green Bay, Chicago, Pittsburgh, Cleveland, Buffalo, New England, Denver, Kansas City. They all play in divisions with other cold-weather teams. Meanwhile, the Seahawks are playing teams from California, Arizona, or Missouri in their division. The climates are significantly different. Climate effects are very real but also very complicated, so that’s best left for another day.

Instead, let’s take a quick look at average home and road margin of victory for each team from 1990 to 2011:

What’s interesting is that if you look at the data just from 2002 to 2011 — which is when Seattle began playing outdoors — the difference between the Seahawks’ home and road margin of victory spikes to 9.6 points (which is third behind Baltimore (9.9) and San Francisco (9.9). Baltimore is an odd one — the Ravens’ struggles on the road are a constant source of material for Mike Tanier — but San Francisco also makes sense as a geographically remote NFC team.

What if we look at just games between teams that are at least 2,000 miles away from each other since 2002? Our sample sizes will shrink, but the Seahawks do move down the list:

My guess is once we control for climate — which is on the real to-do list for this off-season — the gap between Seattle and other teams will continue to shrink. Essentially, a very complex Simpson’s Paradox could explain why Seattle looks to be — and, in many is — much tougher at home.