'It's beyond the model': have Liverpool exposed the limits of xG?
There was just the faintest whiff of schadenfreude about it. A table had been published showing how the Premier League would have finished if it were decided by expected goals. “Look,” a colleague said, “according to xG Manchester City should have won the title by 13 points.” In fact, over the past two seasons, Liverpool should have had 39 points fewer. It pained the colleague to say it, but might Jürgen Klopp have exposed the limits of nerdery?
Seeking to show whether a given shot is a good chance, whether it should have gone in, xG is one of many metrics used in football analytics. It is, however, the first to move into the mainstream. It is featured (after the corner count) on Match of the Day, it pops up in analysis articles and is widespread on Twitter. It has also become a focal point in the debate about how far the magic of football can be distilled into numerical form. So when an xG league table looks so far off the real thing, people start asking questions.
It also provides a good excuse to talk with Ted Knutson, one of the most respected ambassadors for football analytics. An American and a former gambling professional, Knutson is best known for his work establishing Brentford’s analytics-based approach. It was a tough week for him in that regard (of which more later) but his explanation of Liverpool’s secondplaced finish in the xG table is conciliatory towards those who don’t trust the data.
“You can put the difference down to having really great talent,” he says. “Having a great goalkeeper like Alisson will help you give up fewer goals. Part of it is a little bit of luck. But I think there’s stuff Liverpool do that’s not in the expected goals model.
“I’m not going to be specific about that but at a certain point when you have had the second and fourth-best
Premier League seasons ever and the expected goals models don’t really reflect that, maybe you’ve got some stuff going on that is beyond the model.
“Part of [improving the model] is trying to make the data a bit better all the time to reflect that. City I think scored 102 goals and had 35 against but they were blowing out teams. They always keep their foot on the gas. The depth City had coming off the bench during the restart was just insane.
“Liverpool, as good as they are, they don’t have that. So they have learned over the last few years to be able to manage games. Once they get that second goal, especially, they defend well but probably don’t spend so much time exhausting themselves on the pressing side of it.
“I think those two things are reflected in that table. Liverpool do not always put their foot on the gas. But it’s also a little bit how the data and the models don’t reflect some of the edges that they have found and been able to exploit.”
Since xG went large – it first appeared on Match of the Day in 2017-18 – Knutson says there has been a more welcoming approach to analytics within club boardrooms. Knutson’s company, Statsbomb, consults with a number of clubs in the Premier League and several on the continent, including Paris St-Germain.
The issue now, says Knutson, is not scepticism of analytics but using it to make good decisions. “At club level it’s clearly difference in execution [that matters],” he says. “Some clubs are able to get things done and others end up mired in politics behind the scenes. The execution, even with better data, is what’s going to mark out the elite teams from those who are not. That’s really what separated Liverpool – they don’t make mistakes.”
One club characterised by a lack of mistakes, before their goalkeeper David Raya went walkabout in the play-off final at least, is Brentford. With promotion so narrowly missed, however, there will be questions over whether Thomas Frank’s eye-catching side can come again.
“I think the hardest thing for teams to do in football is rebuild, we have seen this over and over again,” Knutson says. “Brentford have sold stars since they first came up to the Championship but they still manage to keep getting better and keep aggregating value.
“They’ve also got a better coach and [as well as being the Championship’s top scorers], Brentford have been great in defence, even through the play-offs till that flukey, goofy goal. I think that they can rebuild, but it’s also the same question for the Leicesters and the Southamptons. Football is hard, transfers are hard. People don’t seem to respect that very much.”