Experienced soccer fans intuitively judge the relative quality of a shot on a number of factors. How close was the shot to the goal? Was the shot on goal or off goal? What kind of angle was between the location of the shot and the goal? Was the shot from a boot or a head? Was the shooter through one-on-one with the goalie or engaged with defenders? Was the shot blocked by a defender before reaching the goalie or goal?
About five years ago expected goals (abbreviated as xG) was introduced as an attempt to quantitatively measure the quality of each individual shot in a soccer match.