Possession of the ball is crucial in football. As Johann Cruyff put it, “Without the ball, you can’t win.” Managers such as Pep Guardiola and Marcelo Bielsa have built successful teams which focus on keeping control of the ball and dominating possession. Other managers, such as Claudio Ranieri during his title-winning season at Leicester City, have focused on using tactics which allow the other team possession in order to catch them off guard with sudden counter attacks. Either way, possession is an important part of understanding why these very different styles work.
Possession stats are designed to show how much of the ball each team has enjoyed. They are generally presented as percentages. If one team in a match had 60% possession, then their opponents had 40%. It was one of the first football statistics to regularly feature in television broadcasts and is now a standard part of post-match analysis.
Possession Measured as Time on the Ball
The most basic way of measuring possession is by recording the amount of time that each team has control of the ball during the match. Control here is used in a broad sense. The ball is generally considered to be in the control of the team to have most recently touched it, regardless of the type of touch.
This way of measuring is simple and only requires somebody watching the game with a couple of stopwatches, manually recording when possession changes. When the ball is out of play, the clock is stopped for both teams. It is a method which is easy to understand. There are, however, major flaws to it.
One major issue is that this method does not measure the extent to which players are actually in control of the ball.
A desperate long clearance from a defender that results in the ball being up in the air for several seconds before dropping to an opponent can count for the same amount of possession as a few quick short passes around the midfield. The amount of control a team has over the ball clearly differs in these two examples, but the old basic way of measuring possession does not factor this in.
How Opta Measure Possession
Analysts developed another method based on the number of passes. If in a game there was a total of 800 passes made by both teams, one team making 480 and the other 320, then the first team was deemed to have had 60% possession. This was the method that the analysts at Opta used until 2017. It gives more of a sense of which team was controlling the ball than relying on time, but there are flaws to it. It does not include types of possession such as dribbling or knocking the ball past an opponent to run on to it.
Since 2017, Opta has used a new metric, based not on the number of passes but the number of ‘possessions.’ They define a possession as starting when a player takes a controlled touch of the ball and ending when he no longer has control due to an event such as an interception or a shot. The number of possessions in a game is then added together to create percentages for each team. Opta send a three-man team to each game they cover. The team apply the possession criteria to judge the extent to which touches are controlled. Their data is used by the BBC, BT Sport, Sky Sports and the Premier League, making it the possession stat you are most likely to see.
Opta’s method of counting the number of possessions is currently dominant in media coverage of the sport, but it is not the be-all and end-all of possession stats.
This is because not all possession is equally important. In 2013, Glasgow Celtic famously beat Barcelona 2-1 in the Champions League after only having 11% possession. Clearly, the Scottish team made better use of the ball than their Spanish opponents.
A way to better understand such matches is to calculate the value of different types of possession. Opta have been developing a metric which they call ‘Possession Value,’ abbreviated to PV. PV aims to measure the probability that a team will score from an individual possession. In this sense, it is similar to metrics such as Expected Goals (xG) and Expected Assists (xA), in that it aims to interpret the importance of an event rather than simply counting the number of times something happens.
Possession stats have traditionally measured the total amount of possession. PV is a stat to assess the value of possession.
The metric uses data from the moments of possession in the build up to a goal, not just the final pass or shot. Using this data, the metric aims to predict the likelihood of any moment of possession resulting in a goal, whether that is a cross into the box or a pass from one defender to another along the backline. By comparing a possession such as a pass or a cross to similar possessions, the stat calculates the percentage of times in which such play resulted in a goal.
Depending on the actions of the player, each possession can increase the PV value or decrease it. This is called ‘Possession Value added’ or PV+. This means the metric can be used to measure the value of each pass or dribble and quantify whether such possessions are more likely to lead to a goal than other possible actions. ‘In this way, it allows us to credit players who may previously have been undervalued by more traditional metrics such as goals and assists,’ says Jonny Whitmore, Senior Data Analyst at Stats Perform.
PV is continuing to be developed and refined. Opta have recently starting using a time-based approach, calculating the probability that the team in possession will score within the next 10 seconds.
The way in which possession is measured has changed considerably in recent years and looks set to continue to develop in complexity. The days in which it is simply measured with a stopwatch are over. Recent developments in data analysis allow for a much more profound understanding of possession.
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