If you take a look back at 2013's expected goal differentials, probably the biggest outlier was MLS Cup runner up Real Salt Lake. Expected to score 0.08 fewer goals per game than its opponents, RSL actually scored 0.47 more goals than its opponents. That translates to a discrepancy of about 19 unexplained goals for the whole season. This year, RSL finds itself second in the Western Conference with a goal differential of a massive 0.80. However, like last year, the expected goal differential is lagging irritatingly behind at --0.77. There are two extreme explanations for RSL's discrepancy in observed versus expected performance, and while the truth probably lies in the middle, I think it's valuable to start the discussion at the extremes and move in from there.
It could be that RSL plays a style and has the personnel to fool my expected goal differential statistic. Or, it could be that RSL is one lucky son of a bitch. Or XI lucky sons of bitches. Whatever.
Here are some ways that a team could fool expected goal differential:
- It could have the best fucking goalkeeper in the league.
- It could have players that simply finish better than the league average clip in each defined shot type.
- It could have defenders that make shots harder than they appear to be in each defined shot type--perhaps by forcing attackers onto their weak feet, or punching attackers in the balls whilst winding up.
- That's about it.
know are pretty sure that RSL does indeed have the best goalkeeper in the league, and Will and I estimated Nick Rimando's value at anywhere between about six and eight goals above average* during the 2013 season. That makes up a sizable chunk of the discrepancy, but still leaves at least half unaccounted for.
The finishing ability conversation is still a controversial one, but that's where we're likely to see the rest of the difference. RSL scored 56 goals (off their own bodies rather than those of their opponents), but were only expected to score about 44. That 12-goal difference can be conveniently explained by their five top scorers--Alvaro Saborio, Javier Morales, Ned Grabavoy, Olmes Garcia, and Robbie Findley--who scored 36 goals between them while taking shots valued at 25.8 goals. (see: Individual Expected Goals, and yes it's biased to look at just the top five goal scorers, but read on.)
Here's the catch, though. Using the sample of 28 players that recorded at least 50 shots last season and at least 5 shots this season, the correlation coefficient for the goals above expectation statistic is --0.43. It's negative. Basically, players that were good last year have been bad this year, and players that were bad last year have been good this year. That comes with some caveats--and if the correlation stays negative then that is a topic fit for another whole series of posts--but for our purposes here it suggests that finishing isn't stable, and thus finishing isn't really a reliable skill. The fact that RSL players have finished well for the last 14 months means very little for how they will finish in the future.
Since I said there was a third way to fool expected goal differential--defense. I should point out that once we account for Rimando, RSL's defense allowed about as many goals as expected. Thus the primary culprits of RSL's ability to outperform expected goal differential have been Nick Rimando and its top five scorers. So now we can move on to the explanation on the other extreme, luck.
RSL has been largely lucky, using the following definition of lucky: Scoring goals they can't hope to score again. A common argument I might expect is that no team could be this "lucky" for this long. If you're a baseball fan, I urge you to read my piece on Matt Cain, but if not, here's the point. 19 teams have played soccer in MLS the past two seasons. The probability that at least one of them gets lucky for 1.2 seasons worth of games is actually quite high. RSL very well may be that team--on offense, anyway.
Unless RSL's top scorers are all the outliers--which is not impossible, but unlikely--then RSL is likely in for a rude awakening, and a dogfight for a playoff spot.
*Will's GSAR statistic is actually Goals Saved Above Replacement, so I had to calibrate.