The numbers behind Arsenal's late-season slump

Alex Livesey

Arsenal began the year looking like title contenders. They're ending the season hanging on by their fingernails for fourth place in the Premier League. What went wrong? I take a look at the underlying statistics to try to piece this together.

With their defeat at Goodison Park two weeks ago, Arsenal lost their grip on fourth place. Everton won their game in hand on the Gunners over the weekend, vaulting them ahead of their rivals for a Champions League place. Fourth now belongs to the Toffees if they win out.

This was supposed to be the season where Arsenal took a real run at the title. They held on to first place in a nearly unbroken run from October through January. Just after the turn of the New Year, they had a fantastic +21 goal difference and a .684 goals ratio. Now it's down to +15 and .583. In this article, I want to take a look inside the numbers to see what went wrong with Arsenal's season, and how they find themselves just hanging on in a tough race for fourth.

Explanation No. 1: Shots and Regression to the Mean

In his discussion of Arsenal's struggles, Ted Knutsen at Stats Bomb pointed to a very simple chart. Arsenal's shot totals. The Gunners have taken 447 non-penalty shots in 33 matches this season, a rate of about 13.5 per match. This is good for only eighth most in the Premier League. Last year the Gunners averaged 15.6 shots per match, and they averaged 16.7 per match the season before that. This is a drop. However, in neither of the 2011-2012 or 2012-2013 seasons were Arsenal among the elite in total shots. They were in fact sixth both seasons in total shots, even though they managed to finish safely third and fourth in the league. So while we do see Arsenal's shot total dropping, we should not expect an Arsene Wenger club to take huge numbers of shots.

As I have previously shown, Wenger's Arsenal create year-to-year the highest-quality chances in the Premier League. They eschew shots from range or shots assisted by crosses, both of which are generally are harder to score. They create higher-expectation shots by working through the middle, in particular by taking shots assisted by through-balls. So to evaluate the quality of Arsenal's attack, it's important to look more deeply than shots totals.

I have here a set of advanced statistics for Arsenal's 2012-2013 season, which break down the Gunners' shots by location on the pitch. I'm looking at total shots, shots from inside the box, and shots from the "danger zone", the close and central areas of the 18-yard box. "CQ" is average chance quality, the estimated chance that an average Arsenal shot has of being score. xG is expected goals based on these estimates. Because different EPL sides have played different numbers of matches, I'm giving the stats on a per-match basis. For a full explanation of the statistics here, see my glossary of advanced statistics.

Arsenal Sh/G Rank SiB/G Rank DZS/G Rank CQ Rank xG/G Rank
Attack 13.5 8th 8.2 6th 5.6 6th .112 3rd 1.52 6th
Defense 11.8 5th 5.8 4th 3.7 2nd .086 6th 1.17 4th

So Arsenal are indeed third in the Premier League in average chance quality (behind Manchester City, of course, and interestingly Manchester United). They are sixth in expected goals. That's not an impressive total, but when combined with good defensive numbers, it's enough to make Arsenal solid fourth place contenders. The Gunners don't look like the eighth best side in the Premier League, but neither are those the stats of a title contender.

Of course, the narrative issue isn't why Arsenal are competing for fourth. It's how Arsenal managed to stay in the title race through January. These statistics suggest Arsenal simply have not been good enough to compete for the title, which raises an obvious question. How were they doing it?

My first explanation, following on Knutsen, is regression to the mean. Arsenal were winning a lot of close matches early in the season, collecting more points than you'd expect based on their shots taken and allowed. This is the sort of trend that is unlikely to continue, and so Arsenal's fall off the pace would be just the normal regression to the mean. A good-not-great side started the season on an unsustainable hot streak, and then as they always do, the hot streak ended.

The following graph shows Arsenal's points per match and expected points per match (based on shot location stats as outlined in the links above).

Ppg_xg_arsenal

For a significant period of this season, Arsenal were playing at an incredible pace of nearly 2.5 points per game. Their underlying stats suggested this was unsustainable, and as the season has gone on, indeed it has not been sustained. The blue line has been moving ever closer to the red line as Arsenal have fallen from the rarefied air of title contention back to their usual place in the top four race. The mostly stable red line suggests that the title challenge was never entirely real, and the club's points per match was always destined to fall back under 2.0.

Explanation No. 2: Injuries and Fitness

While I think simple regression to the mean explains a lot of what has gone wrong for Arsenal, there's still more to it than that. You can see the red line drop over time in the graph, and indeed Arsenal have been playing considerably worse football since the New Year. Bart Schotten at Stats Bomb created an excellent graph showing how Arsenal's form has been falling, adjusted for fixture difficulty, since mid-November.

This is probably related to injuries. In the second half of the season, Arsenal have lost to injury a number of their most important players, in particular Aaron Ramsey and Mesut Özil. Even before the German playmaker went down in January, fans and analysts had noted his decline from a dominant creator to an occasional passenger for Arsene Wenger's side. In a fascinating Sky Sports article making use of the Premier League's tracking data, Adam Bate showed that Özil's physical contributions to the side have been declining since November. He looked specifically at Özil's's "sprints," defined as those times in a match where a player reaches a speed over 25 km/h. While Özil was averaging over 50 sprints per match early in the season, he dropped to 20 or so sprints per match by February.

This reduction in high-intensity work suggests that even before Arsenal's playmaker injured his thigh, he was not working at full fitness. Wenger played Özil match in and match out through the first several months of the season, and he does not appear to have ever recovered. What I want to explore, here, is to what degree Arsenal's decline tracks Özil's.

The inflection point in the graph appears to come in early December, after which Özil never managed 50 sprints in a match again. So I logged the events in Arsenal's matches into three groups. (1) When Özil was not on the pitch, (2) With Özil on the pitch in full fitness, before December, and (3) With Özil on the pitch but not at full fitness. I compared these three sections of the season based on the same expected goals concept from above.

Xgr_ozil

With Mesut Özil on the pitch in full fitness, Arsenal were legitimately excellent with an xGR over .600. This is short a title-winning expected goals ratio (the top teams in the league usually make it to .650 or so), but it's more than good enough to earn a place in the top four. Without their key playmaker, Arsenal have been barely above average, and with a diminished Özil they have been good but not great. Arsenal have not merely played their worst with Mesut Özil, but their form has tracked his fitness as measured by sprint totals.

(I think that the decline in fitness that Bate demonstrates helps explain why, for instance, Özil  does not appear to be a major contributor in this analysis at The Big Ripple.)

As with any plus-minus statistic of this sort, it's impossible to account for every factor. Aaron Ramsey's injury in December is the biggest, but I think this can be considered part of the analysis rather than a confounding factor. Ramsey's absence has coincided with much of the "partial Özil" and "no Özil" stretches, but very little with the "full Özil" period. So we can imagine that some of the above is a Ramsey and Özil effect.

With Aaron Ramsey now back in the squad, and Mesut Özil returning to training after another leg injury, Arsenal may be able to reconstitute the squad that had them cruising through the fall. The statistics suggest this was never really a side that could contend for a league title, but with key players in the team, they were more than good enough to earn a top four position. If those players can return at reasonably full fitness, I think a final turnaround in the season is likewise a reasonable outcome for the Gunners.

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