Expected goals (xG) is a metric used to determine how likely a player is to score a chance and to calculate how many goals a team is expected to score in a match.
The metric was invented in 2012 by Opta’s Sam Green and has become commonplace across football analytics.
In xG, every shot a player has is given a score between zero and one. The higher that number is, the more likely a team is to have scored that particular chance.
This information is calculated from Opta’s historical data from nearly one million shots.
The data model then generates a calculation on the percentage chance the player is likely to have scored in the same scenario.
Other factors taken into consideration include distance, angles, goalkeeper position, the positions of other players, shot type, pattern of play and the previous action.
For instance, a chance that has an xG of 0.2 means that a player had a 20% chance of scoring from that opportunity. A chance with an xG of 0.8 would represent an 80% chance of scoring.
Those xG figures are then added up to determine how a player or team has been performing collectively during a match or season.
The data metric has been criticised by some supporters, pundits and managers, but it is seen as a good indicator of how a player or team has been performing in front of goal.
