The Week in Stats: The numbers behind Liverpool’s defense

This week we see if long-term trends in both shot location and chance creation are continuing this season, and assess the Premier League’s back-lines.

Trend lines

While soccer is the most unpredictable of the major world sports in terms of individual matches, Chris Anderson and David Sally demonstrated in their landmark analytics book The Numbers Game that over the course of an entire league campaign, things are considerably more predictable.

For instance, in 20-team leagues such as La Liga, the Premier League and Serie A, we know that the total number of goals over a full season will be approximately 1,000. Similarly, we can confidently predict that no goals will be scored in about 30 of the 380 matches played and that around 70 will feature just one goal.

There are other areas in those leagues, however, in which we’ve seen a prolonged change over the course of the current decade. Three obvious examples are the percentage of shots being taken from outside the penalty area, the proportion of chances created by long balls and the number of goals assisted by crosses.

In Europe’s top four leagues (La Liga, Bundesliga, Premier League and Serie A), the volume of scoring attempts from outside the box has fallen steadily from 45.26 percent in 2009-10 to 39.77 percent in 2017-18.

During the same period, the percentage of key passes that were long decreased from 24.47 to 17.99, while the proportion of assists that were crosses dropped from 34.42 percent to 29.70 percent.

We’re now far enough into the current campaign to assess whether these long-term trends are continuing this season.

As you can see, the decrease in the proportion of shots coming from outside the box has carried on in 2018-19. Curiously, though, only in the Bundesliga has this ongoing change resulted in a conspicuous decrease in the percentage of goals being scored from outside the area during the same time period.

In fact, in both Serie A and the Premier League the proportion of out-of-box goals is actually higher this season than it was in 2009-10.

The other two trends identified above, namely the fall in long key passes and crossed assists, have both also continued. There is one notable exception here, however: Cross assists are at their highest level in Serie A since 2013.

The 100 club

Even over the course of an entire league season, very few players ever break the 100-shot barrier. Among the big four European leagues since 2009, on only three occasions have more than 10 players in a single division done so. That was Serie A and the Premier League in 2016-17, and Serie A again last season.

Just two players have hit the 200-shot mark during that period, and we can all guess who they are. Lionel Messi has only achieved this once, in 2011-12, while Cristiano Ronaldo managed it on seven occasions. Ronaldo’s 2011-12 total of 264 was the highest for a league campaign.

So far this season, eight players have passed 100 shots domestically.

Harking back

Having looked at the performances of Premier League goalkeepers a couple of weeks ago, here we’ll assess the league’s defenses on a more general level by measuring both the number of chances that each team is conceding and the average quality of those scoring opportunities.

Manchester City have allowed their opponents the fewest shots — just 185 so far — but the xG values of those chances have actually been above the league average this term. Wolves, meanwhile, are conceding the lowest-quality opportunities by some margin.

But it’s Liverpool who have been most successful in restricting the opposition to both low shot numbers and low-quality chances.

As in recent seasons, Burnley have continued to allow opponents high shot volumes but have kept the xG values of those shots low. It just hasn’t worked anywhere near as well for them this time around.

And by these metrics, the much-maligned Arsenal back-line is (i) the closest to average in the league and (ii) performing at a level similar to those of Manchester United and north London rivals Spurs.