Yes, our reader John Collins finally plucked up the courage to ask this all-important question… but I don’t think he is going to like the answer!
While various analytics companies produce their own xG figures, Opta (Stats Perform) is the official data provider for the Premier League. When you see xG graphics on a Premier League broadcast or their official website, you are looking at Opta’s model.
However, “assigning” a value isn’t a manual decision made by a person in a room; it is the result of a statistical machine-learning model (which sounds a lot like Artificial Intelligence!).
How the Values are Assigned
The xG value for a specific chance is determined by comparing that shot to over 300,000 historical shots in Opta’s database. If, historically, shots from a specific situation resulted in a goal 10% of the time, that shot is assigned a value of 0.10 xG.
The model evaluates dozens of variables for every shot, including:
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Shot Location: Distance from the goal and the angle to the goal.
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Body Part: Whether the shot was taken with a foot or the head.
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Type of Play: Was it from open play, a corner, a direct free kick, or a counter-attack?
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Assist Type: Was it a through ball, a cross, or a pull-back?
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Defensive Pressure: How close the defenders and the goalkeeper were to the shooter (in more advanced “tracking data” models).
Key Providers and Their Models
While Opta is the official partner, several other major entities use their own proprietary models to assign values, which is why you might see slightly different xG totals on different websites:
Pro Tip: If you ever see a penalty, most models assign it a flat value of 0.76 xG or 0.79 xG, reflecting the historical conversion rate of roughly 76-79%.