Does
Possession % Matter?
Any
analytically inclined soccer fan (aka you)
is probably well-aware of the limits of possession % as a meaningful metric. In fact, its faults are so numerous and well
documented that many people bring it up solely for ironic purposes. I understand the collective derision, but if
we look at the metric in a deeper way can we glean some interesting
information? I think so.
One
thing that I think does need to be stated is that, despite its limits, there is a relationship between possession %
and points (at least in the EPL – see graph below).
The
causes of this relationship are complex and difficult to disentangle, but
probably the best way to think of possession % is as a symptom of playing winning football as opposed to the cause.
Devin Pleuler has an interesting take on possession as a
defensive weapon.
How
is Possession % Calculated?
Based
on some good work a couple years back by Graham Macaree, we know that the
possession % that the majority of media outlets use is really just a pass ratio. The pass ratio approach is pretty simple:
team possession % = team’s total passes / both teams’ total passes. This
methodology was confirmed to me by an Opta employee. We can debate the merits of this approach
until we are blue in the face, but for many sensible reasons I think it is
probably the best proxy.
Splitting
Possession % into Offense/Defense
Despite
these pass ratios being converted into possession %, not all pass ratios are
created equal. For example, let us
assume that an average EPL match sees 900 passes on average between the two
teams (450 for each team). On this match
day Arsenal outpasses Swansea 600-400 (60%/40%). Across town, West Ham outpasses Crystal
Palace 300-200 (60%-40%). Both Arsenal
and West Ham have the same possession %, but they have achieved them in vastly
different ways. By comparing their
passing #’s to the league average, we can essentially allocate Arsenal and West
Ham’s 20% possession advantage (60%-40%) to an offensive and defensive
component, as demonstrated below. You
start by comparing how many passes each team attempted and allowed and compare
them to the league average. Arsenal, in
this example, was 150 passes above an average offense (600-450). West Ham, by contrast, was 150 passes below
an average offense (300-450). But, West
Ham allowed 250 less passes than an average defense (450-200).
That
was a hypothetical, but what does this approach look like for this year’s EPL?
(stats are a week or two old)
Talk
about a tale of haves/have nots. The
difference between the #1 possession team (Swansea) and the #10 team (Chelsea)
is closer than the difference between Chelsea and the #11 team (Newcastle)! Another thing that jumps out is the
comparison between Southampton and Arsenal; both have similar possession #’s,
but achieve it in a very different fashion: Arsenal with offense and
Southampton with defense. You also might
notice is the larger variance in the offensive component compared to the
defensive component. This makes sense,
as a team might face a variety of passing styles over the course of the year,
but their offensive style is more persistent.
Running some regressions (based on past five years of EPL data – 100
teams) backs this up, as the offensive component has a much stronger
correlation with total possession differential than the defensive correlation. Interestingly, while you would expect a
relationship between the offensive and defensive components, the R2 was
only 0.49, which demonstrates that this exercise of decoupling possession into
offense/defense has some merit.
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