Thursday, January 26, 2012

What's Ailing Arsenal? Diagnosing the Gunners' (Offensive) Weakness With Some Data

Arsenal have had a most unusual season of highs and lows. After a rough start to the campaign, the ship seems to have steadied. Robin van Persie is on pace for a record setting season; his 19 goals so far are only one less than last year's 20 scored by Golden Boot winners Carlos Tevez and Dimitar Berbatov. And iconic legend Thierry Henry has been signed on loan to lend some offensive firepower, experience, and spirit. At the same time, the 36 points Arsenal have accumulated at this point in the campaign are the fewest during Arsene Wenger's reign. This Arsenal season has been anything but boring for outside observers and surely nerve-wracking for supporters.

But what do the numbers tell us about what the Gunners are producing on the pitch? Have Arsenal improved? What have they been doing well? Is it offense, defense, both, or neither?

To start, let's take a look at the trends in the stuff that ultimately matters the most: scoring and conceding goals. The graphs below show the numbers of each, with a best-fitting trend line superimposed to see if there is a pattern to Arsenal's performance over the course of the season to date.

It only takes one quick glance to figure out that Arsenal's offensive season has been one of a significant up and a notable down. Arsenal started the season without much offensive success but saw a significant increase in offensive output all the way to Week 10. But that high point didn't last; instead, it gave way to a steady slide in offensive output all the way to the halfway point of the season. As it stands now, Arsenal's offense is not doing nearly as well as it was 8-10 weeks ago.

In some contrast, Arsenal's inconsistent defensive displays that marked the start of the season seem to have been overcome over the course of the last three months. Halfway through, SzczÄ™sny & Co. managed to steady the ship, put in consistent performances, and produce 7 clean sheets in the process.

Taken together, this produces a mixed picture of where Arsenal stand at this point in the year. On the offensive end of the pitch, after improving significantly two months into the season, and despite van Persie's record-setting goalscoring pace, the Gunners' sharpshooters have been silenced somewhat. In contrast, their defense have allowed only .7 goals on average in the last 10 matches.

So what has happened to Arsenal's offense? To answer that question, lets' look at chance creation and finishing - two of the most critical dimensions of offensive performance for any club.

The next graph plots the number of shots per match during the course of the 2011/12 season to date, along with a trend line superimposed as before. Lo and behold, Arsenal's shot creation did not see any strong trends one way or another. While it fluctuated quite a bit from match to match between around 10 (or less) to 20 and above, it did not reveal any clear drift up or down. While perhaps inconsistent, shot creation hasn't gotten worse.

This pattern stands in noticeable contrast to the Gunners' efficiency numbers, which seem to be largely responsible for the club's declining offensive fortunes. While efficiency went from literally 0 in Week 1 to an almost incredible .4 (or 1 goal for every 2.5 shots) in Week 10, Weeks 11-20 witnessed a slide in efficiency. 

So where does this leave Arsenal? We can put these various pieces of information together in a chart of shot creation and efficiency, with each circle denoting a single match. To help orient readers, I added a blue line denoting the league's median team (the 50% percentile).

This picture isn't pretty if you're an Arsenal fan. The team ended very few matches in the upper right hand quadrant - the quadrant where teams are located if they produce more shots than 50% of the league and convert those chances into goals. Instead, most of the Gunners' matches show them to the left of the vertical blue line - indicating matches in which they created many as well as just a few shots, but also converted these at a rate worse than 50% of Premiership clubs. If you ask me, finishing seems to have become the North London club's Achilles heel, despite van Persie's terrific performances so far.

As with my analysis of Manchester City's performance to date, it's easy to make to much of these relatively simple data points, but they do beg the question of how Arsenal's problem - if properly diagnosed here - can be solved. More consistent chance creation is one - fluctuations between 8 and 24 shots per match are significant - but finishing is the other important piece of the puzzle. For supporters (and analysts), rumors of Robin van Persie handing in a transfer request couldn't come at a worse time for the Gunners.*

* Why that may matter in numeral terms, take a look at Andy Kriebel's excellent analysis here.