Right now, Arsenal are the best football team in England.
But in spite of this fact, a slightly worrying slump has shown up in their data in the period
following the World Cup break: Arsenal’s defensive numbers have been getting worse.
So what is the reason behind this decline?
And does it even matter?
One of the best ways of assessing a team’s defensive
performance is to take a look at the chances that oppositions are creating against them.
We could look at the raw shot data to see how many chances a team are conceding at any
one time but shots can vary in value.
To get a better sense of how well a team are doing defensively, then, it can be good to
look at the expected goal value of the chances they concede.
Expected goal models assign values to chances based on how likely they are to result in
a goal and this can give us a much better idea of defensive performance.
If we look at Arsenal’s performance over the course of the season according to this
metric, we’ll see that things have taken a turn for the worse since the Premier League
resumed after the World Cup
This plot shows you the “expected goals against” figure for Arsenal through the
season on a five game rolling average.
That dip that you can see in the middle is the period just before the World Cup and,
as you can see, Arsenal were at their best defensively for that stretch of the season,
only conceding around 0.5 expected goals against across those games.
What this means is that you would only expect them to concede one goal every other game
on the basis of the quality of chances that they were facing.
As soon as the post World Cup numbers enter the data, though, we see a steep rise, with
Arsenal eventually hitting around 1.50 expected goals against in February in the run of games
which saw them drop points to Everton, Brentford and Manchester City.
There is a sharp drop off after this point, but that is largely due to the Leicester City
game entering the data set.
In that game, Leicester were unable to generate a single chance against Arsenal, resulting
in an expected goals against figure of 0.
That skews the plot slightly and as soon as it drops out of the data, we can see the numbers
start to rise again.
This defensive regression shows up well if you compare the shot map of chances conceded
by Arsenal before and after the World Cup break.
What is most noticeable here are the chances being generated in the six-yard box by Arsenal’s
opponents.
In the 21 games before the World Cup, Arsenal only conceded 7 chances within the six-yard
box, none of which resulted in a goal.
After the break, they seem to have become much more vulnerable in the six-yard box,
conceding more chances and, more importantly, five goals from that area.
So where might this rise in chance quality have come from?
Well, from a tactical point of view you might have expected opposition teams to start sitting
deeper against Arsenal in the second half of the season, spending more time in their
own half and refusing to commit players forward in possession but instead looking to hit them
on the counter.
This shows up in terms of the sorts of goals that Arsenal have been conceding after the
World Cup break.
The amount of goals that they have conceded from deep build up has dropped considerably
in the second half of the season.
This suggests that teams are not trying to progress the ball as much as they were against
Arsenal and are instead looking to hit them more directly through transition.
[Design note: this is accurate as of 31st of March.
They have since conceded a single goal vs Leeds from long distance if you want to add
that to the tally]
However, Arsenal still seem to be as effective at preventing goals in transition after the
World Cup break.
So although teams may have changed their playstyle to face Arsenal, it seems as though the result
has been a net gain to Arsenal defensively.
One area that does stand out on this plot, though, is set piece goals.
Since the World Cup, Arsenal have conceded 8 goals from set pieces compared to 5 from
before.
If we actually look at the expected goals against conceded from set pieces, we can see
a similar decline after the World Cup.
Again, the big peak that we can see in this plot correlates to the run of games in February
where Arsenal dropped a number of points.
Although there is a drop off after this peak again, it’s worth noting that there were
two games in a row where Arsenal conceded no expected goals against from set pieces:
versus Everton and versus Leicester.
But the fact of the matter remains: when Arsenal concede set pieces, they tend to concede expected
goals and, as a result, actual goals.
There’s also a sense in which Arsenal are being impacted by game state as the season
progresses.
What we mean by “game state” is the factors within the game which can affect the way that
both teams play.
For example, a team with a lead may play differently to how they play when they are drawing or
losing.
At the beginning of the season, Arsenal were scoring goals fairly early on with a level
of regularity.
However, as this plot from online analyst Scott Willis shows, the trend line over the
course of the season shows that Arsenal’s first goal has been slowly arriving later
as the season has gone on.
What this means is that Arsenal have not been able to sit on a lead for as long as they
have been doing in the first half of the season, meaning that they are more likely to aggressive
- and thereby defensively open - for greater stretches in the second half of the season.
So should Arsenal fans be concerned about their defensive decline since the World Cup?
Well, the most important factor here is that Arsenal’s expected goals for is still well
in advance of their expected goals against.
But beyond that, it seems natural that, in the course of a season, a team's oppositions
will start to find out where their weaknesses lie and find ways of exploiting them which
will result in a slight decline in their defensive numbers.