Explaining xG or Expected Goals — A technical term which has crept into the lingua franca of football community
If you follow football it’s unlikely that you have never come across the term “expected goals”(abbreviated as xG). xG is a relatively new terminology that has crept into the common football lingo. It is used by football pundits, managers, analysts and fans.
What is xG
Very simply, xG (or expected goals) is the probability that a shot will result in a goal based on the characteristics of that shot and the events leading up to it.
Some of these characteristics/variables include:
- Location of shooter: How far was it from the goal and at what angle on the pitch?
- Body part: Was it a header or off the shooter’s foot?
- Assist Type: Was it from a through ball, cross, set piece, etc?
- Type of attack: Was it from an established possession? Was it off a rebound? Did the defense have time to get in position? Did it follow a dribble?
- Position of defenders/Goal keeper : Was the goalkeeper in position? Was it an open goal or were there a number of defenders between the shooter and the goal? Was the shooter being pressured? Was it a 1v1 situation with the keeper?
That probability is the expected goal total. An xG of 0 is a certain miss, while an xG of 1 is a certain goal. An xG of 0.5 would indicate that if identical shots were attempted 2 times, 1 would be expected to result in a goal.
If you see that chance is described as having an xG rating of 0.20 that means a player would be expected to score from the chance 20 per cent of the time — a one shot out of three would result in a goal.
An open chance in front of the goal from a rebound inside the penalty box has a very high chance of resulting in a goal compared to a shot taken 30 yards with a narrow angle while being pressurized.
Penalty kick as per StatsBomb model has 0.76xG.
How xG is used
xG has many uses. Some examples are:
- Comparing xG to actual goals scored can indicate a player’s shooting ability or luck. A player who consistently scores more goals than their total xG probably has an above average shooting/finishing ability.
- A team’s xG difference (xG minus xG allowed) can indicate how a team should be performing. A negative goal difference but a positive xG difference might indicate a team has experienced poor luck or has below average finishing ability.
- xG can be used to assess a team’s abilities in various situations, such as open play, from a free kick, corner kick, etc. For example, a team that has allowed more goals from free kicks than their xGA from free kicks is probably below average at defending these set pieces.
- A team’s xGA (xG allowed) can indicate a team’s ability to prevent scoring chances. A team that limits their opponent’s shots and more importantly, limits their ability to take high probability shots will have a lower xGA.
xG explained using xG map
The above picture show xG map for Aston Villa vs Liverpool match which resulted in a 7–2 win for the Villans. All shots on target are shown as squares in the map, the size of the square indicates xG. The larger the square, the higher the chance of conversion. Goals are shown in pink color.
Aston Villa had 3.1xG and Liverpool had 1.4xG, both teams managed to outperform their xG.
Aston Villa were undoubtedly, the better team on the field. However, they also have lady luck by their side, three goals of their goals took heavy deflection before ending at the back of the net.
Limitations of xG model
xG model does not take into account the skills of the player who takes the shot.