In the world of sports statistics, there are only a few numbers that are concerned what “should have” happened, not what actually happened. One is errors in baseball, a stat that dates back to the 19th century. Another is a soccer (and sometimes hockey) statistic, one that has only been popularized in the 21st century but has come to dominate much of the talk about the game: expected goals.
Minnesota United is third in the MLS Western Conference standings with 29 points going into Saturday night’s game at Seattle. The Sounders are 11 points behind Minnesota. Yet a glance at the expected goals numbers shows the Sounders have a positive expected goal differential for the season, ahead of the Loons.
What xG, as it’s usually abbreviated, represents is an attempt to assign a number to every shot based on how likely it is to become a goal. It’s become far and away the most-quoted “advanced” statistic in soccer. Coaches will reference it in postgame news conferences, minutes after the final whistle.
The issue is that, just like baseball errors, xG calculations represent subjective judgments — no matter how many computers are used in the calculation. To illustrate this, let’s look at two moments from the Loons’ 2-2 home draw with the LA Galaxy on May 15.
The first came in the first minute of the second half. Loons wingback DJ Taylor crossed the ball for Tani Oluwaseyi, but the ball was slightly behind the striker, 6 yards in front of the Galaxy net. Oluwaseyi, who had a defender on his shoulder, attempted to reach out his right foot behind himself and redirect the ball behind himself and towards the far post — but ultimately the ball rolled 2 yards wide of the goal.
On the other end 15 minutes later, L.A. winger Gabriel Pec made his own pass across the face of Minnesota’s goal, behind the Loons center backs — where he found a wide-open Dejan Joveljíc for what amounted to a tap-in goal for the Galaxy forward.
Now, a subjective observer sees those two plays and knows that Oluwaseyi’s attempt would usually be described as “audacious” — a moment of invention, with little probability of finding the net. Meanwhile, the observer would give Joveljíc — behind the defense, 6 yards out, no defenders around him, just needing to put the ball either side of the keeper — nearly a 100% chance of scoring.
That’s not what xG “sees,” though. It sees two strikers, both attempting a shot with their right foot from the center of the penalty area, 6 yards from goal — among other factors, of course. So if you look at the numbers from Sportec, the league’s data provider, you see that Joveljíc was assigned 0.55 xG and Oluwaseyi 0.58 xG.