Post-Shot xG (PSxG)
Post-shot expected goals adds the trajectory, placement and speed of the strike to the pre-shot model, so the value describes the shot the goalkeeper actually faced.
What it means
A shot aimed into the bottom corner keeps a high value; an identical attempt struck at the keeper falls to a low one. The difference is exactly the save, which pre-shot values cannot see.
Why it matters
It is the correct denominator for judging shot-stopping, because a keeper can only be assessed on the attempts that required a save.
Where it is used
Goalkeeper analysis, and any comparison of keepers whose defences concede shots of different quality.
Where it misleads
It requires tracking or detailed annotation, so a video-only pipeline cannot produce it. Penalties are often excluded, and the exclusion must be stated because it changes the figure materially.
Key points
- It cannot be computed from coordinates alone.
- Pre-shot and post-shot figures are different measurements, not variants.
- Many models exclude penalties from keeper figures.
- Its unit is goals, making it comparable with goals conceded.
- Report shots faced, goals conceded and post-shot values together.
- Shot-stopping above expectation repeats weakly between seasons.
Where two figures for the same thing disagree, the difference is almost always in the definition rather than in the arithmetic.