We all know that home-field advantage exists. I'm sure you've heard of the Seattle Seahawks' record number of false start penalties caused, as well as their perfect home record last year. I'm sure you recall that it took the Sounders playing at the top of their form to finally break Real Salt Lake's home undefeated streak. The advantage exists in basically every sport. It is a universal constant.
Theories for the source of "home-field advantage" vary. It is possibly due to familiarity with the field, court, or pitch. As players play there often, they come to know the bounce of the ball off the Green Monster, the quirks of the lighting and shadows as the sun sets near Safeco, or the best spots for footing in the sand pit that is FedExField. Even if those things bring a negligible advantage, they may provide a significant psychological boost in confidence. A placebo effect, perhaps. Some sports, baseball and hockey, notably, even have rule changes that give slight advantages to the home side.
Often most of the credit for home-field advantage is given to the fans. We all know how loud it can get in the Clink for Sounders games; it gets even louder for the Seahawks. Crowd noise can definitely have an effect on the game. If you can't hear your teammates, you will have problems if you do not have a plan to deal with it. When fans pack The Big House, you can bet there is an intimidation factor, both on opposing teams and on referees.
So we all know it exists, but just how much of it exists? Well, in MLB, teams do slightly better at home, but not significantly better. In the NFL, teams tend to do slightly better. In both sports you can find examples of a few teams that did better on the road, but overall it appears that records are maybe 10-20% better at home (this is a rough eyeballed number). Five of the 8 division winners in the NFL last year either had the same home and away records or were only one win better at home. Hockey is similar to these: teams do better at home, but not significantly better.
Looking at basketball, things are a bit more skewed. Last year in the NBA, teams averaged about 9 more wins at home than on the road. Some of the teams were steady; Brooklyn was about as good on the road as at home, while Orlando was bad pretty much wherever they played. And then you have Denver, who had a winning percentage of .927 at home, but were below .500 on the road. To illustrate how different the NBA is than the other three, only 27% of NBA teams had a winning to .500 record on the road, while in the NHL and MLB, 43% had winning or .500 records, and in NFL, the percentage was 47%.
In MLS, it makes more sense to measure home-field advantage in terms of teams' Home Goal Differential (HGD: goals for minus against at home) versus their Road Goal Differential (RGD: goals for minus against on the road). Last year, only 4 teams had an even or positive road goal differential in MLS. This year that number is just three, after New England fell 3-0 to Kansas City. Only 21% or less of the league is even in the black on the road. In 2012, the average difference between a team's HGD and RGD was a whopping 16.58 goals for the season. That means, with the 34-game season, that teams generally did a half-goal better at home than their "average" (basically how they'd play on a "neutral" field, with the effects of being home or away nullified), and a half-goal worse on the road than their average. This is why, when making calculations in MLS, you generally spot the home team a half a goal before the game even starts.
The numbers this year are slightly more strongly in favor of the home sides. With an average of 23 games played, teams are sitting on a difference of approximately 12 goals between their HGD and RGD, good for a solid .52 goals per game differential (teams are averaging slightly more than half a goal better on the road and half a goal worse at home). By comparison, under the old balanced schedule in 2010, the difference in RGD and HGD was .4 goals per game (I didn't find a good source for the numbers for 2011).
Within the overall numbers, we can pick out some trends. First off, Chivas in 2012 was the only team to be better on the road than at home. And by "better" I mean "not quite as awful." An awful -21 at home versus -13 on the road is not much to celebrate. You can chalk up the majority of the Timbers' improvement to their road form, as they actually have a positive RGD this year. Last year they were at a staggering -25, even though they had a +3 HGD. If John Spencer or Gavin Wilkinson had been able to figure out how to play on the road, maybe Porter would still be at Akron.
The Sounders have one of the best home advantages in the league so far this year, thanks to some big wins at home. They are a slightly below league-average team on the road though. The Sounders are tied for the third largest (worst?) HGD/RGD difference in the league, alongside Montreal and Colorado.
The Galaxy are actually the most "Jekyll and Hyde" team, as they have the best home advantage so far (+14 HGD), yet are -9 on the road, tied for the 4th worst RGD in the league. That's good for a 23-goal difference for their home versus away form. That's more than a goal a game of difference, and it is almost entirely due to their defense. At home they have only allowed 4 goals. On the road though? 26!! That's .4 GAA at home, and 2.0 GAA away! If the Sounders have to face another annual 3-0 deficit in the playoffs, I might actually feel confident if we lost 3-0 on the road to LA then came to Century Link for the second leg. LA has allowed 3 goals against Vancouver, Salt Lake, Dallas and San Jose, and also lost 5-0 to New England earlier this year. They additionally allowed 2 goals against Toronto, Portland, and Colorado. The Galaxy do not have a good defense on the road. They have a terrible defense on the road.
In other little tidbits, Montreal has a pretty bad RGD like LA (with -9), which helps explain their downturn in form. Only three teams have a negative HGD. San Jose somehow has 30 points, despite having a -10 GD, tied with TFC, who are sitting on 20 points. SJ loses really bad on the road, then does OK at home. Meanwhile, Chivas and DC are really, really bad. Both have negative overall GDs in the 20's, and are still in the teens in points.
So what does all of this mean, if anything? Well, if you're a betting fan, I'd suggest spotting the home side a healthy margin. Teams can and do win on the road, but usually only when they are significantly better than the other team. It also seems that the unbalanced schedule meant to cut down on travel has not really changed the nature of teams playing worse on the road. A lot more goes into the travel than the pure distance involved, so perhaps teams find the travel just as tiring. It could also mean that the biggest advantage to the league of reduced travel distance is actually the decreased cost of travel.
Of course, the key takeaway is that home-field advantage is very strong in MLS, perhaps stronger than in any other sport in America. Why do I think that is? Personally, I think MLS supporters--meaning fans, from casuals to ultras--are the best sports fans in America. The data supports it.