By Kevin Minkus (@kevinminkus)
D.C. United's defense, according to advanced metrics, is not very good. As of this weekend, their Expected Goals Against (xGA) sit at 15.7, their Total Shot Ratio (TSR) is .415, and they've allowed 156 shots in 11 games. According to these stats, they should find themselves near the bottom of the Eastern Conference standings. But, miraculously, they're not. Instead, they're at the top of the standings, with 21 points through 11 games. They've allowed only nine goals, tied for fewest in the league.
Some consider this to mostly be a product of luck; after all, they've outperformed their xGA by nearly seven goals, and their PDO is at 1058. Given the quality of shots they've faced this season, the probability of them allowing nine goals or fewer is just three percent. These numbers suggest they are due for a regression sometime soon. But there's a problem. The numbers said the same thing last season, and that regression never happened. In 2014, D.C. United finished first in the East with 59 points, third most in the league. They had just a two percent chance of outperforming expected goals allowed by the margin that they did (about 12 goals). A question on many MLS analysts' minds is “How?”
At least part of this over-performance probably can be attributed to luck (and maybe unconscious biases in the data). You can't ever really rule it out entirely. But a much greater part of it has a simple, straightforward explanation: they actually do defend well, even though it doesn't show up in the stats in ways we might expect it to.
The chart below shows expected goals against minus goals against for 2014, as well as the probability of at least that level of over or under performance occurring. Teams with a highly positive xGA-GA are “lucky”, teams with a highly negative xGA-GA are “unlucky”. The right-most column is the percentage of shots against that are off target or blocked.
|Team||xGA-GA||Chances of Allowing at Least that Many Goals||% of Shots Blocked or Off Target|
The two teams that over-performed xGA the most, D.C. United and Real Salt Lake, also were in the top four for highest percentage of off target or blocked shots against. The correlation between these two variables is pretty high for 2014. For the years 2011 to 2014, it isn't quite as high, but is still significant. That is, the teams that over-perform expectations the most generally force higher than expected numbers of off target and blocked shots. This, of course, makes sense; shots that miss or get blocked can't become goals. United is once again forcing a lot of misses this season, as their percentage of shots that are off target or blocked stands at about 62 percent.
This, too, could merely be the product of luck. None of off target percentage, block percentage, or off target plus block percentage is, in general, particularly repeatable from season to season. However, off target plus block percentage is generally a good indicator of save percentage, at least for teams with the highest percentages (for whom it is less likely to be luck):
From these numbers, and from watching them play, the logical conclusion is that D.C. United pressures shooters on the ball, and gets defenders behind the ball, to the extent that they force opposing teams to shoot poorly, resulting in missed shots, blocked shots, and shots that are easy saves for the goalkeeper. Put another way, their defensive pressure causes teams to shoot at percentages below what expected goals models would predict. That D.C. United's defense leads to a high save percentage also means that their PDO will be a poor measure of how “lucky” they are.
As further evidence that this is not just luck, teams that consistently force a lot of blocked and missed shots also tend to be teams that allow a high proportion of shots from crosses. D.C. United and Real Salt Lake, 2014's two most over-performing teams, were also the two teams that faced the highest percentage of shots off crosses. This suggests those teams are making the conscious decision to pack the box and pressure shots, at the cost of allowing space out wide.
Most teams can be considered “good” defensively by the number of shots they give up. A low number of shots typically means a low number of expected goals, which in turn means a low number of goals. Other defenses are considered “good” because they only give up low percentage shots. They may give up a lot of shots, but, because those shots aren't likely to go in, they have low expected goals totals, and they don't allow many actual goals.
D.C. United's defense, as I've tried to show, takes a third way. They give up shots, from good positions, but because of their abnormally high defensive pressure, those shots are more likely to miss the target or be blocked or saved, and therefore expected goals models overstate their chances of going in. Because of this, most metrics will systematically mis-evaluate the team's defense. This was the case last season, and it is once again the case at the start of this season. Presumably, expected goals models that incorporate defensive positioning would more accurately describe the team's defensive performance, but until the day comes when that data is made publicly available, we won't know for sure.