I expect this is valuable but surely it should take into account who is taking the shot? If it's me or Leo Messi then the key factor in whether it's likely to result in a goal is probably not the angle or the type of attack.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:
Every shot is compared to thousands of shots with similar characteristics to determine the probability that this shot will result in a goal. 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 .5 would indicate that if identical shots were attempted 10 times, 5 would be expected to result in a goal.
- 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?
- Type of pass: 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?
There are a number of xG models that use similar techniques and variables, which attempt to reach the same conclusion.