The Week in Stats: Messi continues to defy logic
By Warren Pegg
Having looked last week at expected assists, here we’ll do the same for expected goals (xG), identifying which players and teams are over- and under-achieving when it comes to chance conversion.
Perhaps xG’s most useful characteristic is its predictive power — indeed, it’s a much better indicator of future goal-scoring performance than the actual number of goals that a team or player has scored. So it’s possibly the best statistical gauge of luck, both good and bad, that we have.
With a couple of notable exceptions — elite teams tend to over-achieve in xG terms season after season, as do a handful of individual coaches and players — this means we can confidently predict that xG performances will revert to the average, usually over the course of a season.
So we can assume that that the majority of both the teams and players who are currently over- and under-performing in xG terms are unlikely to maintain that form, be it good or bad, for the rest of the campaign.
On average, in the Understat model teams have been under-achieving by 0.6540 xG during the season to date. In other words, clubs are scoring slightly fewer goals overall than the model predicts.
You’ll see above that no team in Europe’s top four leagues has over-performed in xG terms this season more than Borussia Dortmund, who have 54 Bundesliga goals from 39.82 xG of chances. As discussed last week, this just translates as: Lucien Favre:
Arsenal’s Unai Emery doesn’t share Favre’s long history of over-achieving in xG terms, which is why it was pretty much universally – and accurately – predicted that Emery’s side wouldn’t be able to continue their extraordinary early-season run of form.
Valencia have suffered the greatest misfortune in front of goal, finding the net just 24 times (the seventh-lowest total in La Liga) from an xG of 39.94 (the fourth-highest in the league).
It’s a remarkable state of affairs given the attacking talent in their squad and also demonstrates that the club have been right to stick with coach Marcelino. In fact, expected points stats indicate that they’ve been the second-best team in Spain’s top tier this term.
Such misfortune seems to be blighting the Valencian Community as a whole, because Valencia’s near-neighbors Villarreal are the second-biggest under-achievers here. Their surprising relegation battle at least appears to be more down to very bad luck than very bad play.
Again, in expected points terms, their performances would on average have put them ninth in the La Liga rankings, instead of the 19th position they currently occupy. So we should expect Villarreal to rise up to mid-table before the season’s end.
Moving on to look at non-penalty expected goals (NPxG) for individual players in the top four leagues, Jamie Vardy is the Premier League’s worst under-achiever, having found the net five times from an NPxG of 8.57. Meanwhile, Cardiff’s Sean Morrison has failed to score at all despite having 3.26 NPxG of chances.
The presence of Gabriel Jesus in the same list again raises the prospect that Manchester City could be scoring even more goals, which is slightly unnerving, especially for Liverpool fans. If anyone’s surprised not see Crystal Palace’s Christian Benteke in there, he’s sixth in the rankings, with an NPxG under-achievement of 2.68.
It will come as no surprise to see that Borussia Dortmund’s Paco Alcacer is the Bundesliga’s biggest NPxG over-achiever, as nobody believed that he could keep scoring at such a preposterous – and deliriously entertaining – rate.
Even though the Spaniard has cooled off in recent weeks, his 12 goals from just 5.99 NPxG of chances remains a league-high over-performance. Meanwhile, Robert Lewandowski’s stand-out under-performance – 13 goals from 19.51 NPxG – seems emblematic of Bayern’s difficult season.
Next. The Week in Stats: Silva’s teammates are letting him down. dark
Having already detailed the xG woes of Valencia and Villarreal at team level, it’s hardly a shock that four of their players are among La Liga’s top-six under-achievers.
Conversely, the presence of three of the Celta Vigo squad in the league’s top-ten NPxG over-performers reinforces the belief that they’re fortunate even to be as high as 16th in the table.
Lionel Messi, of course, just continues to defy the numbers in the way he’s always done.
Finally, in Serie A, Cristiano Ronaldo has been scoring lots of goals because he takes a ludicrously high number of shots — 148 in the league alone so far, 51 more than any other player in the Italian top flight — rather than because he’s been any good at finishing. His conversion rate in NPxG terms is currently the fourth-worst in the ltalian top tier, just 13 goals from 16.13 NPxG of opportunities.
And Milan is another center of wayward shooting this season, with AC Milan’s Hakan Calhanoglu and Matias Vecino of Inter having the two of the three highest NPxG totals in Europe’s top four leagues without scoring.
