This column provided by Northwestern University seniors Sam Stejskal and Shashank Churukanti and the good folks at soccer analytics start-up, Chimu Solutions. Take a trip on over to Chimu Solutions (kind of like an Opta on steroids.)
We see possession numbers thrown around all the time in the soccer world.
It’s easy to understand why: the possession stat is simple to calculate and even simpler to understand.
Another reason for the number’s ubiquity: It seems like a good measure of which team will win a match. Hold the ball longer than your opponent and the chances of you scoring more goals – and thus winning – would seem to go up.
But is this actually true? Do teams that hold the ball longer really have a better chance of winning? Do possession stats tell us anything at all?
To find out, we studied possession numbers from every single MLS game this season. We gathered data from all 287 regular season matches through October 12th, first tabulating the goal differential for each game. Here are the results of that tabulation:
We then proceeded to calculate the difference in possession of the two teams.
For draws, we were simply interested in the absolute value of difference in possession, whereas in matches with winners, we subtracted the possession percentage of the losing team from the percentage of the winning team. A positive number means that the winning team held the possession advantage, while a negative number indicates that the losing team held more of the ball.
After this, we calculated the mean value of the possession difference for all margins for victory.
If having the ball more often really does lead to scoring more goals, then winning by a larger margin should mean a larger advantage in possession.
Simply put, we expect a positive correlation between possession difference and margin of victory.
The following graph disproves this hypothesis:
Apart from the sole 5-goal game (New York’s thrashing of Toronto FC in early July), it is evident that such a positive correlation has not been present thus far in the MLS regular season. In fact, after omitting the lone outlier, the data shows that the only margin of victory that has flirted with the 50 percent possession mark this year is the two-goal win.
However, for our purposes, there is a problem with using mean values. While they disprove any positive correlation between higher margin of victory and having the majority of possession, mean values are too general. They don’t give us any insight about specific possession data measured against wins and losses.
The below graphs give us this specific data, showing that winning teams have had less possession than their opponents in 112 of the 184 games (61 percent) that have ended in a win.
The following diagrams show the possession numbers for games with one- and two-goal margins of victory.
*remember that a negative number implies that the losing team had an advantage in possession (i.e. if the winning team had a 40-60 disadvantage in possession, the measure for the game would be -20)
The data for games with three- and four-goal margins of victory is similar.
All of this data means that the popular notion that having the ball more increases the likelihood of winning hasn’t held true in the 2011 MLS regular season.
We see this with each of the current top four teams in the league: L.A., Seattle, Salt Lake and Dallas. Each of these teams holds less of the ball in wins than in ties or losses, with fourth-place FC Dallas sticking out the most, holding the ball just 45.6 percent of the time in their 14 wins while having it 53 percent of the time in their 11 losses and 46.8 percent of the time in their seven draws.
The possession data for all of the top four teams is shown below:
Real Salt Lake is a particularly interesting case. They didn’t lose the possession battle until their 21st game – a 3-0 win over New York on August 6th. Since that game, RSL has had less of the ball than their opponents five times – all wins – and more of the ball in six matches – all losses.
All of this data – along with RSL’s curious case – begs the question: why do teams win more when they are out-possessed?
There are several possible reasons. First and foremost, teams that grab a large early lead might tend to sit back and defend, allowing their opponents to hold more of the ball after they’ve built a comfortable advantage.
A good example of this is D.C.’s 4-1 victory over Real Salt Lake on September 24th. D.C., powered by Dwayne De Rosario’s record-breaking first half hat trick, took a 4-0 lead into the half. They didn’t do much attacking after the break, sitting back and allowing RSL to dictate the game in the second half. This tactic – which, of course, is totally reasonable and completely understandable – was partially responsible for RSL’s huge 66.1-33.9 percent possession advantage.
Other possible reasons why MLS teams win more when they have less of the ball could be counterattacking play and discrepancies in finishing ability.
Really, there could be any number of reasons for this interesting phenomenon. The point is, holding an advantage in possession is not a good predictor of which team will win a game.
At Chimu Solutions, we recognize this – and are looking to come up with metrics that better predict performance. We use a network model to statistically analyze player and team performance, calculating the importance of each player within the context of their team’s network and properly rewarding players for their contributions to attacking and defending movements.