Expected Goals (xG) is a metric used in football to measure the probability of a shot resulting in a goal. It is a statistical measurement of the quality of goalscoring chances and the likelihood of them being scored. An xG model uses historical information from thousands of shots with similar characteristics to estimate the likelihood of a goal on a scale between 0 and 1. Not all xG models take into account the same factors, but most consider distance to goal, angle to goal, body part, type of assist or previous action, goalkeeper position and status, the position of all attackers and defenders in frame, and shot impact height.
xG can be used to get a better idea of underlying team quality beyond current results, highlighting teams who are over or under-performing their expected numbers and whose results may soon begin to change. Expected Goals totals can be used to determine when a player might be in the midst of a hot or cold streak in front of goal, while looking at average shot quality (xG/shot) can highlight players taking good or bad shots. xG has many uses, such as comparing xG to actual goals scored to indicate a players shooting ability or luck, assessing a teams abilities in various situations, and indicating a teams ability to prevent scoring.
However, xG has some weaknesses. It is a shot-based model and shots only account for 1.5% of all events in a football match, so the other 98.5% of the game is unaccounted for. Also, it wouldnt be smart to use xG on a single-match basis because the game state can affect the matchs final xG, and football is a low-scoring sport with a lot of variance and luck, and certain match events may dramatically affect xG.
