Every shot is priced by what shots like it are worth: its make probability from location and context, and the free throws its zone draws league-wide that season. A player's number is the gap between what he produced and what those looks were worth, which is why it is called over expected.
The model is leak-free: nothing that happens after a shot is an input to valuing it, and the fit is cross-validated by season so a player's own results never price his own shots.
What this does not measure: with shot location taken out it still cannot separate contact-seeking skill from the kind of shot within a zone or from officiating, and it is not evidence of bias in any direction.
Every game's page shows its final score and what each side's shots were worth: field goals priced by how often shots like them go in, free throws at the shooter's season rate. The gap between the two is not a luck number, because misses bring putbacks and a team that falls behind takes more shots and draws more free throws.
Over Expected is a basketball shot-value platform covering NBA, EuroLeague, EuroCup, Liga ACB, Lega Basket Serie A, Basketball Bundesliga, Greek Basket League, ABA Liga, WNBA.