Sporting exceptional at home; RSL lame on the road
/It is true that Sporting has had trouble getting points at home. SKC earned 30 points at Sporting Park this year, good for 13th in a league of 19 teams. Based on that information alone, some will argue that Sporting is not a good home team. One of those people is Simon Borg, who justifies his viewpoint by pointing out that SKC lost five times at home, as though that matters. It doesn't.
I've shown that past points simply don't correlate well to future points. With information like shot ratios and expected goal differentials (xGD), points are essentially a meaningless indicator of team ability---or at the very least, a meaningless predictor. I see "predictor" and "indicator" as near-synonyms in this instance, but you may not. Regardless, Sporting's home points total should not even be considered in the discussion of who will win on Saturday. Why not? In addition to out-shooting its opponents in every single home game this season, here is how SKC did relative to the league in xGD this season:
| Team | GF | GA | GD | xGF | xGA | xGD | "Luck" |
| LA | 32 | 8 | 24 | 32.1 | 11.0 | 21.1 | 2.9 |
| SKC | 29 | 15 | 14 | 28.7 | 11.6 | 17.1 | -3.1 |
| PHI | 23 | 17 | 6 | 28.8 | 16.0 | 12.8 | -6.8 |
| NYRB | 32 | 15 | 17 | 26.8 | 15.4 | 11.4 | 5.6 |
| SEA | 28 | 15 | 13 | 26.5 | 15.6 | 10.9 | 2.1 |
| COL | 28 | 16 | 12 | 25.3 | 14.5 | 10.7 | 1.3 |
| HOU | 23 | 16 | 7 | 28.0 | 18.9 | 9.1 | -2.1 |
| RSL | 31 | 16 | 15 | 24.6 | 16.4 | 8.2 | 6.8 |
| CHI | 28 | 19 | 9 | 27.0 | 19.4 | 7.6 | 1.4 |
| SJ | 23 | 13 | 10 | 28.8 | 21.2 | 7.6 | 2.4 |
| POR | 28 | 11 | 17 | 23.9 | 16.6 | 7.2 | 9.8 |
| CLB | 19 | 13 | 6 | 25.6 | 18.8 | 6.8 | -0.8 |
| NE | 29 | 15 | 14 | 22.2 | 17.4 | 4.8 | 9.2 |
| MTL | 31 | 19 | 12 | 25.1 | 20.5 | 4.6 | 7.4 |
| FCD | 28 | 21 | 7 | 24.2 | 19.8 | 4.4 | 2.6 |
| VAN | 32 | 18 | 14 | 23.7 | 19.5 | 4.2 | 9.8 |
| DCU | 16 | 27 | -11 | 23.5 | 21.3 | 2.2 | -13.2 |
| TOR | 22 | 21 | 1 | 18.5 | 19.1 | -0.6 | 1.6 |
| CHV | 16 | 28 | -12 | 18.9 | 26.0 | -7.1 | -4.9 |
SKC has a decent goal differential at home, but more importantly, it has the second-best expected goal differential at home. xGD is an excellent predictor of future success, and a better indication in my mind of true team skill.
Borg goes on to talk about the "road warriors" from Salt Lake City:
"They love playing on the road. Playing at home is too much pressure; they do it better when they're away from home."
No team is better on the road than at home, but whatever. RSL did tie for third in MLS this season with 22 away points earned, but again, we don't care. RSL out-shot it opponents in just five of 17 road games (29.4%), and, well this:
| Team | GF | GA | GD | xGF | xGA | xGD | "Luck" |
| SKC | 16 | 15 | 1 | 19.3 | 18.2 | 1.1 | -0.1 |
| SJ | 11 | 29 | -18 | 20.5 | 21.3 | -0.8 | -17.2 |
| LA | 20 | 30 | -10 | 18.5 | 20.2 | -1.7 | -8.3 |
| FCD | 18 | 28 | -10 | 19.9 | 23.6 | -3.7 | -6.3 |
| HOU | 17 | 23 | -6 | 21.7 | 25.4 | -3.8 | -2.2 |
| POR | 25 | 22 | 3 | 18.9 | 23.5 | -4.6 | 7.6 |
| COL | 15 | 22 | -7 | 19.9 | 24.9 | -5.1 | -1.9 |
| NYRB | 24 | 24 | 0 | 19.5 | 25.6 | -6.1 | 6.1 |
| PHI | 19 | 26 | -7 | 19.4 | 26.7 | -7.3 | 0.3 |
| NE | 19 | 21 | -2 | 16.1 | 23.7 | -7.6 | 5.6 |
| CLB | 22 | 33 | -11 | 17.1 | 26.0 | -8.9 | -2.1 |
| SEA | 11 | 27 | -16 | 17.9 | 27.2 | -9.4 | -6.6 |
| CHI | 18 | 30 | -12 | 20.5 | 30.0 | -9.5 | -2.5 |
| MTL | 19 | 29 | -10 | 16.1 | 26.0 | -9.9 | -0.1 |
| VAN | 21 | 23 | -2 | 16.6 | 27.7 | -11.0 | 9.0 |
| TOR | 6 | 25 | -19 | 15.9 | 27.3 | -11.4 | -7.6 |
| RSL | 25 | 25 | 0 | 17.4 | 29.7 | -12.3 | 12.3 |
| DCU | 5 | 28 | -23 | 11.9 | 26.2 | -14.3 | -8.7 |
| CHV | 12 | 38 | -26 | 12.0 | 28.9 | -16.9 | -9.1 |
Real Salt Lake finished 17th in the league in expected goal differential on the road. Ouch. The fact that their actual goal differential was tied for third in MLS means very little, since xGD makes for a much better Nostradamus.
Unless expected goal differential completely falls apart in home-away splits---which is not likely---we can conclude that Sporting is a good home team, and RSL is a bad away team.
Our current model gives Sporting 72 percent probability of a win. An xGD model---which we don't use yet because we only have one season of data---increases those chances to 88 percent. There is a lot of evidence that Sporting is the better team, and that home field advantage still applies to them. Regardless of Saturday's outcome, those two statements are still well supported.
*Note that these goal statistics do not include own goals, which is why my figures may differ slightly from those found at other sites.