Not quite the same old Arsenal

The narrative surrounding Arsenal has been strong this week, with their fall to fourth place in the table coming on Groundhog Day no less. This came despite a strong second half showing against Southampton, with Fraser Forster denying them. Arsenal’s season has been characterised by several excellent performances in terms of expected goals but the scoreline hasn’t always reflected their statistical dominance. Colin Trainor illustrated their travails in front of goal in this tweet.

I wrote in this post on how Arsenal’s patient approach eschews more speculative shots in search of high quality chances and that this was seemingly more pronounced this season. Arsenal are highly rated by expected goal models this season but traditional shot metrics are nowhere near as convinced.

Analytical folk will point to the high quality of Arsenal’s shots this season to explain the difference, where quality is denoted by the average probability that a shot will be scored. For example, a team with an average shot quality of 0.10 would ‘expect’ to score around 10% of their shots taken.

In the chart below, I’ve looked at the full distribution of Arsenal’s shots in open-play this season in terms of ‘shot quality’ and compared them with their previous incarnations and peers from the 2012/13 season through to the present. Looking at shot quality in this manner illustrates that the majority of shots are of relatively low quality (less than 10% chance of being scored) and that the distribution is heavily-skewed.

ShotQualFor_Arsenal

Proportion of total shots in open-play according to the probability of them being scored (expected goals per shot). Grey lines are non-Arsenal teams from the English Premier League from 2012/13 to the present. Blue lines are previous Arsenal teams, while red is Arsenal from this season. Data via Opta.

In terms of Arsenal, what stands out here is that their current incarnation are taking a smaller proportion of ‘low-quality’ shots (those with an expected goal estimate from 0-0.1) than any previous team by a fairly wide margin. At present, 59% of Arsenal’s shots reside in this bracket, with the next lowest sitting at 64%. Their absolute number of shots in this bracket has also fallen compared to previous seasons.

Moving along the scale, Arsenal reside along the upper edge in terms of these higher quality shots and actually have the largest proportion in the 0.2-0.3 and 0.3-0.4 ranges. As you would expect, they’ve traded higher quality shots for lower quality efforts according to the data.

Arsenal typically post above average shot quality figures but the shift this season appears to be significant. The question is why?

Mesut Özil?

One big change this season is the sustained presence (and excellence) of Mesut Özil; so far this season he has made 22 appearances (playing in 88% of available minutes) compared to 22 appearances last season (54%) and 26 matches in his debut season (63%). According to numbers from the Football in the Clouds website, his contribution to Arsenal’s shots while he is on the pitch is at 40% compared to 30% in 2014/15. Daniel Altman also illustrated Özil’s growing influence in his post in December.

Özil is the star that Arsenal’s band of attacking talent orbits, so it is possible that he is driving this focus on quality via his creative skills. His attacking contribution in terms of shots and shot-assists is among the highest in the league but is heavily-skewed towards assisting others, which is unusual among high-volume contributors.

Looking at the two previous seasons though, there doesn’t appear to be any great shift in Arsenal’s shot quality during the periods when Özil was out of the team through injury. His greater influence and regular presence in the side this season has probably shifted the dial but quantifying how much would require further analysis.

Analytics?

Another potential driver could be that Wenger and his coaching staff have attempted to adjust Arsenal’s tactics/style with a greater focus on quality.

Below is a table of Arsenal’s ‘volume’ shooters over the past few seasons, where I’ve listed their number of shots from outside of the box per 90 minutes and the proportion of their shots from outside the box. Note that these are for all shots, so set-pieces are included but it shouldn’t skew the story too much.

Arsenal_OoB_Shots_TableThe general trend is that Arsenal’s players have been taking fewer shots from outside of the box this season compared to previous and that there has been a decline proportionally for most players also. Some of that may be driven by changing roles/positions in the team but there appears to be a clear shift in their shot profiles. Giroud for example has taken just 3 shots from outside the box this season, which is in stark contrast to his previous profile.

Given the data I’ve already outlined, the above isn’t unexpected but then we’re back to the question of why?

Wenger has mentioned expected goals on a few occasions now and has reportedly been working more closely with the analytics team that Arsenal acquired in 2012. Given his history and reputation, we can be relatively sure that Wenger would appreciate the merits of shot quality; could the closer working relationship and trust developed with the analytics team have led to him placing an even greater emphasis on seeking better shooting opportunities?

The above is just a theory but the shift in emphasis does appear to be significant and is an interesting feature to ponder.

Adjusted expectations?

Whatever has driven this shift in Arsenal’s shot profile, the change is quite pronounced. From an opposition strategy perspective, this presents an interesting question: if you’re aware of this shift in emphasis, whether through video analysis or data, do you alter your defensive strategy accordingly?

While Arsenal’s under-performance in terms of goals versus expected goals currently looks like a case of variance biting hard, could this be prolonged if their opponents adjust? It doesn’t look like their opponents have altered tactics thus far based on examining the data but having shifted the goalposts in terms of shot quality, could this be their undoing?

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Shooting the breeze

Who will win the Premier League title this season? While Leicester City and Tottenham Hotspur have their merits, the bookmakers and public analytics models point to a two-horse race between Manchester City and Arsenal.

From an analytics perspective, this is where things get interesting, as depending on your metric of choice, the picture painted of each team is quite different.

As discussed on the recent StatsBomb podcast, Manchester City are heavily favoured by ‘traditional’ shot metrics, as well as by combined team ratings composed of multiple shooting statistics (a method pioneered by James Grayson). Of particular concern for Arsenal are their poor shot-on-target numbers.

However, if we look at expected goals based on all shots taken and conceded, then Arsenal lead the way: Michael Caley has them with an expected goal difference per game of 0.98, while City lie second on 0.83. My own figures in open-play have Arsenal ahead but by a narrower margin (0.69 vs 0.65); Arsenal have a significant edge in terms of ‘big chances’, which I don’t include in my model, whereas Michael does include them. Turning to my non-shots based expected goal model, Arsenal’s edge is extended (0.66 vs 0.53). Finally, Paul Riley’s expected goal model favours City over Arsenal (0.88 vs 0.69), although Spurs are actually rated higher than both. Paul’s model considers shots on target only, which largely explains the contrast with other expected goal models.

Overall, City are rated quite strongly across the board, while Arsenal’s level is more mixed. The above isn’t an exhaustive list of models and metrics but the differences between how they rate the two main title contenders is apparent. All of these metrics have demonstrated utility at making in-season predictions but clearly assumptions about the relative strength of these two teams differs between them.

The question is why? If we look at the two extremes in terms of these methods, you would have total shots difference (or ratio, TSR) at one end and non-shots expected goals at the other i.e. one values all shots equally, while the other doesn’t ‘care’ whether a shot is taken or not.

There likely exists a range of happy mediums in terms of emphasising the taking of shots versus maximising the likelihood of scoring from a given attack. Such a trade-off likely depends on individual players in a team, tactical setup and a whole other host of factors including the current score line and incentives during a match.

However, a team could be accused of shooting too readily, which might mean spurning a better scoring opportunity in favour of a shot from long-range. Perhaps data can pick out those ‘trigger-happy’ teams versus those who adopt a more patient approach.

My non-shots based expected goal model evaluates the likelihood of a goal being scored from an individual chain of possession. If I switch goals for shots in the maths, then I can calculate the probability that a possession will end with a shot. We’ll refer to this as ‘expected shots’.

I’ve done this for the 2012/13 to 2014/15 Premier League seasons. Below is the data for the actual versus expected number of shots per game that each team attempted.

xShots_historic_AVB

Actual shots per game compared with expected shots per game. Black line is the 1:1 line. Data via Opta.

We can see that the model does a reasonable job of capturing shot expectation (r-squared is at 0.77, while the mean absolute error is 0.91 shots per game). There is some bias in the relationship though, with lower shot volume teams being estimated more accurately, while higher shot volume sides typically shoot less than expected (the slope of the linear regression line is 0.79).

If we take the model at face value and assume that it is telling a reasonable approximation of the truth, then one interpretation would be that teams with higher expected shot volumes are more patient in their approach. Historically these have been teams that tend to dominate territory and possession such as Manchester City, Arsenal and Chelsea; are these teams maintaining possession in the final third in order to take a higher value shot? It could also be due to defenses denying these teams shooting opportunities but looking at the figures for expected and actual shots conceded, the data doesn’t support that notion.

What is also clear from the graph is that it appears to match our expectations in terms of a team being ‘trigger-happy’ – by far the largest outlier in terms of actual shots minus expected shots is Tottenham Hotspurs’ full season under André Villas-Boas, a team that was well known for taking a lot of shots from long-range. We also see a decline as we move into the 2013/14 season when AVB was fired after 16 matches (42% of the full season) and then the 2014/15 season under Pochettino. Observations such as these that pass the ‘sniff-test’ can give us a little more confidence in the metric/method.

If we move back to the season at hand, then we see some interesting trends emerge. Below I’ve added the data points for this current season and highlighted Arsenal, Manchester City, Liverpool and Tottenham (the solid black outlines are for this season). Throughout the dataset, we see that Arsenal have been consistently below expectations in terms of the number of shots they attempt and that this is particularly true this season. City have also fallen below expectations but to a smaller extent than Arsenal and are almost in line with expectations this year. Liverpool and Tottenham have taken a similar number of shots but with quite different levels of expectation.

xShots_Historic_plus_Current

Actual shots per game compared with expected shots per game. Black line is the 1:1 line. Markers with solid black outline are for the current season. Data via Opta.

None of the above indicates that there is a better way of attempting to score but I think it does illustrate that team style and tactics are important factors in how we build and assess metrics. Arsenal’s ‘pass it in the net’ approach has been known (and often derided) ever since they last won the league and it is quite possible that models that are more focused on quality in possession will over-rate their chances in the same way that focusing on just shots would over-rate AVB’s Spurs. Manchester City have run the best attack in the league over the past few seasons by combining the intricate passing skills of their attackers with the odd thunder-bastard from Yaya Touré.

The question remains though: who will win the Premier League title this season? Will Manchester City prevail due to their mixed-approach or will Arsenal prove that patience really is a virtue? The boring answer is that time will tell. The obvious answer is Leicester City.