What are/is Expected Goals?
Expected goals (xG) is a model-based metric used to isolate the evaluation of play-driving and chance-creation/suppression ability from things a player cannot control such as bounces, quality of goaltender, etc. The models make use of the public data tracked by the NHL. The NHL tracks every unblocked shot attempt (Fenwick) and collects over 100 pieces of information per unblocked shot attempt (shooter, location on ice, type of shot, etc.). Analytics nerds leverage this historical data to train the data-driven expected goals models.
The models are applied to new shot attempt data to calculate the «probability of goal» for those shot attempts. For example, a one-timer from the slot might have an expected goals value of 0.25, because that type of high-danger opportunity results in a goal 25% of the time. Common jargon is, «that scoring chance was worth 0.25 expected goals.» (Note: I made up the number for that example, not every slot shot is worth 0.25 expected goals.)
