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. 

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