Apr 23, 2012, 1:36 PM EDT
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.
Apr 1, 2015, 7:47 PM EDT
This is a strange one: footballer admits to thinking about tweet to critic while still game in which he scored.
Apr 1, 2015, 6:22 PM EDT
A serious knee injury at the business end of the season? Terrible news, though Alaba hopes to return for a pair of cup finals.
Apr 1, 2015, 5:58 PM EDT
The first of CONCACAF’s five WCQ rounds is complete, and there’s a world-famous name leading one Caribbean minnow into the next round.
Apr 1, 2015, 5:15 PM EDT
The USMNT kicks off its 2015 Gold Cup campaign in 97 days. What’s the roster going to look like? Who’s the surprising “ins” and “outs”?
Apr 1, 2015, 3:30 PM EDT
April Fools’ Day jokes — everybody’s got them, and while most aren’t funny, a few people came through with good ones today.
Apr 1, 2015, 3:00 PM EDT
One little magician has been sublime all season. He’s our consensus pick, but who do you choose?
Apr 1, 2015, 2:28 PM EDT
Ribery is a big fan of Hazard. Should the Chelsea wing-wizard replace the Frenchman at Bayern?
Apr 1, 2015, 1:58 PM EDT
PST’s writers select their coach of the year candidate with the end of season awards coming up fast.
Apr 1, 2015, 1:20 PM EDT
Listen to the latest MiB podcast, here.
Apr 1, 2015, 12:58 PM EDT
We select our Young Player of the season. Do you agree with the consensus selection?
Apr 1, 2015, 12:25 PM EDT
Sterling has spoken out about his current contract situation. It’s fascinating stuff.
Apr 1, 2015, 11:20 AM EDT
PST’s writers pick their team of the season, so far… Agree? Disagree?
Apr 1, 2015, 10:35 AM EDT
Which players did themselves the world of good in March for the U.S.? Here’s a gander.
Apr 1, 2015, 9:45 AM EDT
Chairman of Bayern Munich, ECA, believes Blatter is ready to change his ways.
Apr 1, 2015, 9:10 AM EDT
Spurs winger scores stunner to silence doubters. Now, can he keep it up?
Apr 1, 2015, 8:01 AM EDT
Here are the latest rumors involving Premier League clubs.
Mar 31, 2015, 9:26 PM EDT
As bizarre as this is to read, Michael Bradley didn’t register a single helper for the States until 2011. In the Klinsmann era, he has 12.
Mar 31, 2015, 8:07 PM EDT
It was out of character for Altidore, who hadn’t received a red card for club or country since he was with AZ Alkmaar in 2012.
Mar 31, 2015, 7:17 PM EDT
With the deal Porto have turned another big profit. See this incredible list below, as the Portuguese club giants have made close to $423 million.
- What will Jurgen Klinsmann’s USMNT roster look like for 2015 Gold Cup? 1
- April Fools! The top gags that actually fooled soccer fans on this day 4
- PST’s Premier League award predictions: Here’s our Player of the Season 0
- PST’s Premier League award predictions: Here’s our Coach of the Season 0
- PST’s Premier League award predictions: Here’s our Team of the Season 3
- Who were biggest winners, losers from USMNT’s March friendlies? 3