Different models have different basic events that define them.
Shot based expected goals models have all goal attempts as their baseline unit.
XG2 models to rate keepers have attempts on target.
And non shot xG models are framed around possessions.
For the latter we'd like to know more about an individual possession. we can measure the speed, (see previous primer), but we'd also like to know what's happening during a possession.
Usually that's passes and runs with the ball that are designed to probe defenses and create space.
We therefore look at how many significant events occur per possession.
If there's lots of passes and lots of runs, you might be chasing shadows.
If there's just mainly passing, you're probably better able to keep a good defensive shape.
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