If you come in from a certain angle, you can hype this evening's DC United-Sporting KC game as the Eastern Conference's clash of the week. The two teams enter this game tied for the second seed with two of the best goal differentials in the conference. With DCU playing at home, and Sporting missing half its team, the edge would appear to go to United. But not so fast. Despite being inseparable by points, DCU and Sporting are about as far apart as two teams can be by Expected Goal Differential. Sporting sits atop the league at +0.62 per game,* while DCU is ahead of only San Jose with -0.33. If we look to even gamestates---during only those times when the score was tied and the teams were playing 11-on-11---the chasm between them grows even wider. Sporting's advantage over DCU in Even xGD is more than 1.5 goals per game.*
To this point, as early as it is in the season, I have found that winners are best predicted by Even xGD, rather than overall goal differential. Though the sample size of shots is smaller for each team in these scenarios, the information is less clouded by the various tactics that are employed when one team goes ahead, or when one team loses a player.
Of course, Sporting will be missing the likes of Graham Zusi, Matt Besler, and Lawrence Olum, as they have for the past three games. The loss of those key players has mostly coincided with their current four-game winless stretch, and it would be tempting to argue that they are not in form. However, over those last three games, Sporting overall xGD is +0.27 per game,* and its Even xGD is +0.68.*
Making predictions in sports is generally just setting oneself up for failure---especially in a sport where there are three outcomes---but I will say this. Sporting is likely better than the +180 betting line I'm seeing this morning.
*I use the phrase "per game" for simplicity, but xGD is actually calculated on a per-minute basis in our season charts. Per game implies per 96 minutes, which is the average length of an MLS game.