We uncover definitions of the latest advanced data metrics in football.
Expected goals (xG)
Expected goals (xG) measures the quality of a chance by calculating the likelihood it will be scored from a particular position on the pitch during a particular phase of play. This value is based on several factors from before the shot was taken.
xG is measured on a scale between zero and one, where zero represents a chance that is impossible to score and one represents a chance a player would be expected to score every single time.
We know that a chance from the half-way line isn't as likely to result in a goal as a chance from inside the box. With xG, we can actually quantify how likely a player is to score from each of these situations.
For example, suppose the chance from inside the box with a given set of pre-shot characteristics was worth 0.1 xG. This means that an average player would be expected to score one goal from every ten shots in this situation or 10 per cent of the time.
Some of the most important variables are listed below:
- Distance to goal
- Angle to goal
- One-on-one
- Big chance
- Body part (such as header or foot)
- Type of assist (such as through ball, cross, pull-back)
- Pattern of play (such as open play, fast break, direct free kick, corner kick or throw-in)
Some situations are particularly unique and these are modelled independently. Penalties are given a constant value, corresponding to their overall conversion rate (0.79 xG), direct free-kicks have their own model and headed chances are valued differently for set-pieces and open play.
Expected goals on target
Whereas xG measures the quality of the chances that a side makes, expected goals on target (xGOT) builds on this to tells us what a team managed to do with these chances. Essentially, xG is a pre-shot model, while xGOT is a post-shot model.
The xGOT model is built on historical on-target shots and includes the original xG of the shot but also the goalmouth location where the shot ended up. It gives more credit to shots that end up in the corners compared with shots that go straight down the middle of the goal. This model is only for on-target shots, given there is a zero per-cent chance a shot will result in a goal if it is off target.
- xG is a pre-shot model, whereas xGOT is a post-shot model.
- At shooter level, we can better understand if goalscoring has been due to good finishing over a sustained period or as a result of poor goalkeeping.
- Goalkeeper performance can be better understood when comparing goals to xGOT faced.
Goals prevented
Given that expected goals on target (xGOT) measures the probability of shots on target resulting in goals, the only factor preventing them being scored is the goalkeeper. A shot that is worth 0.3 xGOT has a 30-per-cent likelihood of being scored but it is also a shot that has a 70-per-cent chance of being prevented by the goalkeeper.
This means that we can predict how many goals a goalkeeper would be expected to concede, based on the quality of the shots that they faced. It allows us to directly credit to goalkeepers for their ability to prevent goals, irrespective of their team's defensive strengths.
For example, during the Premier League 2019/20 season, Sheffield United's Dean Henderson was one of the standout performers. Based on the quality of the shots on target the Manchester United loanee faced (39.4 xGOT), the average goalkeeper would have been expected to concede over 39 goals. Given he only conceded 32 goals (excluding penalties and own goals), we can credit Henderson with preventing more than seven goals with his saves, which was the fourth highest in the Premier League that season.
Expected assists (xA)
Expected assists (xA) measures the likelihood that a completed pass will become a goal assist. It considers several factors, including the type of pass and end point and length of pass. Adding up a player or team's expected assists gives us an indication of how many assists a player or team should have had, based on their build up and attacking play.
Expected pass completion (xP)
Just as expected goals (xG) predicts the likelihood of a shot being scored, the xP framework models the probability of a pass being completed by taking information about the pass and the current possession.
- Start location of the pass
- Angle of the pass
- Distance of the pass
- Whether the pass was played with a foot or by head
- Whether the pass was a cross, in open play or during a set-piece
- Whether the pass was along the ground or airborne
- Other factors based on the overall sequence before the pass was made
Carries
The total number of times a player runs with the ball at their feet for a distance of five metres or more.
- Carry distance: The total distance a player has carried the ball at their feet, in metres.
- Carry progress distance: The total distance a player moves the ball upfield with their carries, in metres.
Defenders bypassed
The total number of defenders goal side of the ball carrier at the beginning of the touch event, minus the number of defenders goal side at the end of the touch event. This value is negative if the defensive team recovers players behind the ball carrier during the carry event.
Off-the-ball runs
A sustained off-ball movement with intensity by an offensive player. A run must start with intensity before the pass targeting the runner is, or would be, played.
Conditions:
- The runner becomes open to receive a pass
- The runner takes at least four steps in the run
- The runner forces a reaction from the defence, which means at least one of the following:
- The defender of the runner changes
- The shape of the defence is affected in a nontrivial way
- Space is created for a teammate to receive the ball
High turnovers
The number of sequences that start in open play and begin 40m or fewer from the opponent's goal.
Player pressures
A defensive event attributed to an individual player. Pressure can be applied directly to the ball carrier (direct pressure) or indirectly to potential receivers (indirect pressure). Pressure is characterised by the defender's sustained speed, direction of movement and distance from the opponent.
Team presses
A coordinated series of pressures by two or more defenders. The defensive team must be in an 'organised defensive state' - a 'team press' cannot occur when the defensive team is in a 'transition defensive state'.
Counter press
A pressure that begins no more than two seconds after losing possession but excludes instances directly after a restart in play.
Pressed sequences
The number of times a team breaks opposition passing sequences at three or fewer passes via a defensive action, within 40 metres of the opposition's goal.
Opposition passes allowed per defensive action (PPDA)
PPDA is the number of opposition passes allowed outside of the pressing team's own defensive third, divided by the number of defensive actions by the pressing team outside of their own defensive third.
High defensive actions
The number of defensive actions by the pressing team in the opposition's defensive and middle thirds.
Sequences and possessions
A sequence starts with a player making a controlled action on the ball, such as a pass, to form a passage of play, which belongs to one team and is ended by a defensive action, stoppage in play or a shot.
Possessions are a combination of one or more sequence in a row belonging to the same team. That possession continues until the opposition wins the ball back by making a controlled action on the ball themselves.
For example, if the previous sequence ended with a defender kicking the ball out of play (uncontrolled action), then a second sequence begins - but it remains part of the original 'possession'.
- Passes per sequence: The average number of passes per sequence.
- Sequence time: The average time per sequence, in seconds.
Open-play 10+ passing sequences
The number of open-play sequences, which comprise of 10 or more passes.
Absolute width per sequence
The maximum distance a team gets from the centre of the pitch per sequence, in metres.
Directness
The percentage of total distance covered upfield per sequence - calculated as upfield progress divided by total distance.
Direct speed
The number of metres per second progressed upfield in open-play sequences - the median is used to remove extreme values.
Progress
The distance a team moves upfield per sequence, in metres - calculated as the end x-coordinate minus the start x-coordinate.
Build-up attacks
The number of open-play sequences comprising of 10 or more passes and either ends in a shot or has at least one touch in the opposition box.
Direct attacks
The number of open-play sequences that start just inside the team's own half and has at least 50 per cent movement towards the opposition's goal and ends in a shot or a touch in the opposition box.
Transition opportunities
The total number of opportunities a team in possession has to attack a defence in transition.
Starting distance
The average distance a team starts open-play sequences from their own goal, in metres.
Field tilt
The share of possession a team has in a game, factoring only touches or passes in the attacking third.