Apr 23, 2012, 1:36 PM EST
One of the amazing statistics to come out of last Wednesday’s UEFA Champions League match was the possession number. Barcelona was reported by UEFA was having held the ball 72 percent of the time, an amazing figure against a club of Chelsea’s caliber. For those who have tried to find significance to correlations between possession and victories, the number must have been both remarkable and beguiling. After all, Barcelona lost, giving more credence to the hypothesis’ main qualm: What if one team doesn’t care about holding the ball?
The next day, the possession story got even more confusing. Supreme stat overlords Opta reported that Chelsea had only managed 20 percent of the ball. What? Even less time in possession? How freakish is this data point going to get?
That, however, is not the story. At least, it’s the story in light of what Graham MacAree notes at Chelsea fan site We Ain’t Got No History. As he’s found out, Opta seems to be miscalculating possession; or, better put, Opta is not reporting a number consistent with the normal expectation for a possession stat.
The normal expectation: When one team has the ball, they’re in possession. I think we can all agree on this, right? This still leaves a lot of gray area. For example, who gets credit for possession when midfield chaos leaves neither side in control? Does one team get possession on a goal kick, when most goal kicks lead to 50-50 midfield challenges? And more broadly, what happens when play is dead but the game clock is running?
I’ve always assumed this is like a chess clock. When one team controls the ball, you hit a button that sends their dials turning. When the other fully regains possession, you hit a button. One clock stops. The other starts running. Those in between moments? They’re governed by one rule: Until possession changes, don’t touch anything.
That, apparently has nothing to do with Opta’s calculations. In fact, Graham’s research suggests Opta doesn’t even run a clock, which may be why they never report possession in terms of time. Instead, the relation between reported possession and total passes suggests Opta just uses passes. As Graham found out, if you take a team’s pass attempts a divide it by the game’s total attempted passes, you have Opta’s possession stat.
What does this mean? Let’s take a totally fake scenario. Barcelona plays three quick passes before trying a through ball that rolls to Petr Cech. It all takes four seconds, while Petr Cech keeps the ball at his feet for eight seconds before picking it up, holding it for five seconds, then putting it out for a throw in, which takes eight more seconds to put back into play.
Despite Barcelona having possession for only four of those 25 fake seconds, they’d have 80 percent of Opta’s possession (three good passes plus one bad, while Chelsea had only Cech’s unsuccessful pass). A logical expectation of a zero-sum possession figure would have that as either 16 percent or (if you credit the time out of play as Barça’s, since they’d have the ensuing throw) 48 percent Barcelona’s. Or, if you do a three-stage model (that’s sometimes reported in Serie A matches), you’d have 16 percent Barcelona, 52 percent Chelsea, and 32 percent limbo/irrelevant.
Of the three methods of reporting possession, Opta’s bares the least resemblance to reality; or, it’s the one that deviates furthest from what we expect from a possession stat.
Ironies being a thing these days, there are two here. First, Opta is the unquestioned leader in soccer data management. How could this happen?
Second, Opta isn’t trying to hide their methods. In fact, they’ve published a post on their site detailing not only their practices but their motivations and research, an investigation that found their approach “came up with exactly the same figures (as time-based methods) on almost every occasion.”
You would think two curmudgeons like Graham and myself would have found this, right? Graham had a reader point it out to him, while a representative from Opta magnanimously pointed me to the piece without the seemingly necessarily indignation of explaining how a Google search works. After all Graham’s work and head scratching – after my lack of work and similar head-scratching – we could have just gone to Opta’s site.
“We try to be as transparent as possible with this stuff,” Opta said when I asked them about it. Certainly, they should be commended being so up front about their methods. After all, they’re a business that makes money off their work. They don’t need to give away their secrets.
But that’s a secondary issue. The main one: Why is a data house like Opta, reputed as the industry standard, taking this short cut? Or, why haven’t they renamed their measure? Granted, the perception that it is a shortcut may have more to do with our expectations than their intent, though based on their defense in the post, it’s clear they do see this as an accurate way of describing possession.
Still, the number they publish is completely redundant to the raw passing numbers also distributed. Why put the measure out at all if not to check a “possession stat” box on a list of deliverables?
Opta’s possession stat shouldn’t be cited in reporting, and if it is, the word “possession” shouldn’t be used to describe it. Reader expectations for anything labeled “possession” are drastically different than what Opta’s producing. The number is confusing to the point of being misleading. It’s becoming counter-information because of its poor packaging.
Even though Opta’s post on the topic is 14 months old, most will be surprised to hear this “news.” It’s disconcerting for anybody who is hoping a SABR-esque revolution’s on the horizon. Almost all of the huge volume of data to which we have access has been useful, but where people are expecting something akin to linear weights to be published tomorrow, we can’t even agree on the terms (let alone the significance of them).
Graham probably puts it better:
I’m completely fine with keeping track of passing volume – I’ve done it before myself. What’s frustrating, from an analyst’s point of view, is that we’re being sold a dud. A statistic that ostensibly measures possession measures something that is not possession, and gets repeated as authoritative anyway.
And people wonder why football statistics don’t get taken very seriously.
Mar 7, 2014, 8:08 PM EST
Away goals now rule playoff matchups, and tiebreaker rules for playoff spots have changed priorities.
Mar 7, 2014, 7:28 PM EST
Looking for yet another reset button, Clint Dempsey will indeed begin the 2014 MLS season with Seattle’s matchday squad.
Mar 7, 2014, 6:34 PM EST
After incidents last weekend, Brek Shea and his teammate have both apologized to fans after lashing out at abusive supporters.
Mar 7, 2014, 5:57 PM EST
PST’s experts weigh in and predict the MLS Cup champs and the final standings:
Mar 7, 2014, 5:44 PM EST
For a year and a half the Ligue 1 side won’t be able to acquire any players.
Mar 7, 2014, 5:13 PM EST
Will the Sounders finally win an MLS Cup in 2014? What will happen to Sigi Schmid if it doesn’t happen?
Mar 7, 2014, 4:00 PM EST
Major League Soccer starts it’s big leap next season. In 2014, the league will enjoy the end of an era.
Mar 7, 2014, 3:40 PM EST
Where and how to watch every PL game live during Week 29:
Mar 7, 2014, 3:08 PM EST
With both newcomers and familiar faces, the 2014 MLS season features a jumble of quality at the front of attacks across the league.
Mar 7, 2014, 2:45 PM EST
Former coach Pia Sundhage, in charge of the USWNT when their streak began, guided Sweden to a 1-0 victory in the Algarve Cup – the USMNT’s first defeat in two years.
Mar 7, 2014, 2:03 PM EST
It once seemed incomprehensible that Leo Messi could leave Barcelona. But is it a possibility?
Mar 7, 2014, 1:15 PM EST
Can Kansas City start their defense of the MLS Cup off on the right foot, grabbing a win at Seattle?
Mar 7, 2014, 12:30 PM EST
When Jonathan Walters converted his match-winner, there was only one song on the Stoke supporters minds: Delilah.
Mar 7, 2014, 11:43 AM EST
The regular MLS referees will be locked out of the weekend’s opening games, meaning the league will turn to qualified replacement officials.
Mar 7, 2014, 11:14 AM EST
The Manchester United manager penned a letter to season ticket holders praising their loyalty and promising the club will be great once more.
Mar 7, 2014, 10:25 AM EST
Juventus insist the midfielder isn’t for sale, but with at least three big names sniffing around, will the Italian giants be tempted?
Mar 7, 2014, 9:40 AM EST
Will either Cardiff or Fulham get the three points they so desperately need?
Mar 7, 2014, 8:43 AM EST
Don’t worry, Leo fans: Messi’s reaction during Argentina’s match against Romania was perfectly normal.
Mar 7, 2014, 7:51 AM EST
Will Spurs be able to break the spell José Mourinho has cast over Stamford Bridge?
Mar 7, 2014, 7:01 AM EST
Will United really pay such a high price for young Luke Shaw?
- PST writers predict the 2014 Major League Soccer standings, do you agree? 1
- After monumental rebuild, Seattle Sounders target MLS Cup title. Can Sigi Schmid bring success? 3
- 2014 Season Preview: Welcome to the last year of MLS 2.0 4
- WATCH: Premier League TV Schedule – Week 29 0
- MLS Preview: Seattle Sounders vs. Sporting Kansas City 0
- WATCH: Stoke City fans erupt into “Delilah” chant during win over Arsenal 0
- Everton is Focused on Beating Arsenal
- Moyes and Martinez Back Cleverley
- Luis Suarez Continues to Say the Right Things
- RSL 2014 Season Preview
- Portland Timbers Season Preview
- Toronto FC Season Preview
- Duvall, Oyongo & Stevenson Sign Contracts with NY Red Bulls
- Moyes Writes Open Letter to Man United Fans
- San Jose Earthquakes: Players To Watch In 2014 Season
- A Good Start Would Go A Long Way for the Chicago Fire