Despite all the advances made in terms of data analysis in recent years, football remains an extremely difficult sport to analyze comprehensively.
There are several reasons why it’s more complicated than any other major team sport, but perhaps the simplest is the lack of goals. It’s the lowest-scoring sport in the world, which means teams aren’t always rewarded for doing the right things, and dominance doesn’t always translate into success.
One consequence is that the form of teams can vary dramatically between good and bad without them doing too much differently. And while it’s easy to work backwards from the outcome and turn heroes into villains, it’s often difficult to understand precisely what has changed.
Perhaps the most difficult club to analyze lately has been Liverpool. Given that they are, of Europe’s elite, apparently the club that has put the most emphasis on data analysis, this seems somewhat ironic. Or maybe it’s not ironic at all and Liverpool are doing things that others haven’t learned to measure. But it’s worth crunching the numbers to describe how difficult it has been to gauge Liverpool’s performance compared to what we expected.
Jurgen Klopp’s Liverpool have been inconsistent (Photo: Marc Atkins/Getty Images)
“Expected” now has two different meanings. There is the literal meaning of expected: how we expected Liverpool to perform at the start of each season. And then there’s the new analytical meaning of expected: Liverpool’s expected goals (xG) numbers. Either way, Liverpool have been a very strange team.
Below are the figures from Jurgen Klopp’s six full seasons in charge of Liverpool. The “points” are your actual points. His “PSE” is the preseason expectation of his eventual point tally, taken from SportingIndex’s spread betting markets, which serves as a good summary of the overall expectation. The xG column shows his expected points number based on the expected number of goals from the 38 games, as calculated by Understat.
Clearly there is a lot of variation here and what is interesting is the relationship between the different categories. So here’s another table: first it shows how Liverpool have outperformed or underperformed relative to their pre-season expectations, then relative to their xG figures. The final column shows how they fared in terms of xG relative to preseason expectations.
|Difference PSE||xG diff||PSE – difference xG|
Some of these numbers are extraordinary.
First, in the last four seasons, Liverpool have never come within 13 points of their pre-season expectations, exceeding SportingIndex analysis three times, once falling well short.
Second, Liverpool have never underperformed relative to their xG difference, and over those four seasons are 44 points better than xG would suggest.
Third, there are pretty big swings in the relationship between preseason expectations and his eventual xG numbers.
At this point, it’s worth showing Manchester City’s equivalent numbers. The point here is not to compare the two sides in terms of performance, but in terms of performance relative to expectations.
Here are City’s raw numbers…
And here, more relevantly, the relationship between the city’s numbers.
|Difference PSE||xG diff||PSE – difference xG|
So while Manchester City have not been entirely predictable, their performances have been easier to predict than Liverpool’s.
The biggest difference between their pre-season expectations and their actual points total is 12 points, fewer than in each of Liverpool’s four campaigns.
The difference between their point total based on xG and their actual point total has been seven or less, while Liverpool’s difference has been as much as 25.
And perhaps most interestingly, preseason expectations always closely match their xG difference.
Essentially, City are a predictable team, but Liverpool are considerably more chaotic, which generally matches their pitching styles.
What’s happening this season? The pre-season betting markets suggested Liverpool would win by 86 points, now they imply it will be around 71 points. The xG figures suggest they could have gained a single extra point than they actually have.
In other words, this is similar to two seasons ago, when they underperformed relative to preseason expectations, but not relative to their xG. This time, of course, they don’t have the excuse of back injury problems.
Let’s take that 71-point finish for this season as gospel. It would be a drop of 21 points compared to last season. But it would actually be just six points below last season’s preseason expectation. That would be a higher number than two years ago, in terms of actual points and xG. And perhaps most peculiarly, it would be just three points short of what the xG would suggest they should have achieved since their title-winning season.
Liverpool’s 2020 Premier League triumph was a statistical over-performance (Photo: Laurence Griffiths/Getty Images)
And so, without ignoring Liverpool’s current slump, it’s worth remembering that their numbers in recent seasons have been a remarkable overperformance, which was always completely unsustainable. In fact, if you compare their numbers more directly to those of their regular title rivals, not once have pre-season expectations put Liverpool ahead of Manchester City, and not once has their xG figure been higher to City’s, even in their title-winning season. .
This certainly leaves us with more questions than answers. But perhaps a fair conclusion to draw is that, while Manchester City are a very logically constructed team that largely perform roughly as expected, Liverpool are very different. Liverpool have always been about excitement, chaos, things that don’t quite add up, whether under Klopp or in the 15 years before him: think Alaves in 2001, AC Milan in 2005 or West Ham in 2006. There’s something about Liverpool that defies normality.
At times this season, Klopp has seemed unable to provide a full answer as to why Liverpool are struggling so badly. It’s not satisfying for the fans, but it’s fair enough in a way. Similarly, it has often been difficult to explain why Liverpool have been so good.
(Top photo: Oli Scarff/AFP via Getty Images)