Since 2008, 100 different MLS squads have taken the field and 96 of them have played better at home than on the road (measured by goal differential). The average home team over the past five and a half years has a positive goal differential of +0.47 goals a game.
Home field advantage is a big deal. But in a league as diverse in geography and supporter culture as MLS is— think 5,000 people at a Chivas USA game vs. 40,000 at a Seattle Sounders game—you would expect some teams to have a greater advantage than others.
In the table below you can see every team’s home field advantage for every season from 2008-2013. These years were chosen as they represent the contemporary Soccer Specific Stadia-version of MLS. Home field advantage is measured by taking a team’s home goal differential and subtracting their road goal differential. This is not necessarily the most accurate possible measure (doesn’t adjust for SOS, etc.) but it represents the most sensible proxy.
The variation from year to year can be quite staggering. The LA Galaxy, despite playing in the same stadium, went from having basically no home field advantage in 2009 and 2010 to having the largest differential so far in 2013. Because of this variability, among other reasons, only limited conclusions can be drawn. For example, although Montreal, Vancouver, and Portland all have great home field advantages—both statistically and by the eye test—the limited sample size for all three in terms of games played precludes any broad declarations.
Perhaps the one factor that does appear to have an effect, as Ted Knutson correctly hypothesized, is altitude. Of the 14 teams that have been in the league from 08-13, the two teams that play at altitude—RSL and Colorado—have the best and third best home field advantages, respectively. Somewhat curiously, the New York Red Bulls have the second best home field advantage over that same time frame, though it noticeably depreciated once they moved into Red Bull Arena in 2010.