Does Possession % Matter?

Any analytically inclined soccer fan (a.k.a. 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 the ubiquitous  ironic mentions of “but what about possession?” every time Barcelona loses have (mostly) stopped.  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 there is a relationship between possession % and points (at least in the EPL – see graph below).

epl poss v points

 

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, though of course sometimes it is the cause! Confusing! A must read on this subject is  Devin Pleuler’s  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

Not all pass ratios/possession % 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 particular 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 % (60%), 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, were 150 passes above an average offense (600-450).  West Ham, by contrast, were 150 passes below an average offense (300-450).  But, West Ham makes up this difference by allowing 250 less passes than an average defense (450-200).

possession differential example

 

That was a hypothetical, but what does this approach look like for this year’s EPL? (stats are two weeks old)

epl pos diff

 

Talk about a tale of possession 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 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 strong relationship (R2  > 0.7) between offensive and defensive components, the R2 was only 0.49, which I think demonstrates that this exercise of decoupling possession into offense/defense has some merit.

offense defense rsquared

 

offense v defense

  • Errorr

    How accurate is that type of possesion to really let us get an idea of how much Our intuitive definition of traditional possession be.? Is the assumption that the time distribution of passes is Gaussian? Is this footballs version of the efficient markets hypothesis?

    • alex olshansky

      Perhaps it is our version of EMH. I didn’t want to get too into the theory behind possession% in this piece, but that Graham Macaree piece I linked to I think has a nice discussion on the topic. And what is our traditional view of possession? Is it the “stop-clock” idea? If so, there are problems with that approach too.

  • Adrian

    Hi,

    Was just trying to look at this over 2 seasons but my results look weird. eg (stats all from Squawka) Southampton ave 55% possession this year and made 11540 successful passes which would mean their opponents made 9442. Cardiff ave 47% possession making 8100 passes and their opponents therefore 9134. This would suggest that Cardiff were better at stopping their opponents passing than Southampton??!

    Please help

    Thanks

    • alex olshansky

      Adrian,

      Yes, based on that scenario you describe it would appear that Cardiff were better at stopping passes than Southampton. I can’ t account for any discrepancies between Squawka and Whoscored (my source), but I can say from my end that this discrepancy is due to I have Southampton at 58.5% and Cardiff at 45.3% possession, respectively. Based on my information applied to those passing totals you would have Southampton allowing 8,186 passes and Cardiff allowing 9,780. It is a weird discrepancy though, I wonder if Squawka calculates possession differently? It is very possible.

      • Colin Trainor

        Most possession sources are simply based on number of passes.

        Squawka do calculate possession differently as they claim that they do actually use possession times instead of just pass numbers. So that would be why WS and SQ would show different possession numbers.

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