The Proxy for Possession
In hockey, you cannot legally hold the puck for long periods of time like you can in basketball. The puck is constantly deflected, dumped, and chased. Because measuring exact "time of possession" with a stopwatch is impossible, analytics experts needed a proxy metric to determine which team was actually controlling the flow of the game.
They invented Corsi. Named after Jim Corsi (a former goaltender coach), Corsi simply counts every single shot attempt directed at the net—including goals, saves, missed shots, and blocked shots.
The Theory of Corsi
The fundamental mathematical theory is that if your team is shooting the puck at the opponent's net, the opponent cannot be shooting at yours. Therefore, shot attempts (Corsi) correlate almost perfectly with offensive zone possession time.
The Formula
A player's Corsi For Percentage (CF%) measures the ratio of shot attempts their team generates while they are on the ice versus the shot attempts the opponent generates.
CF% = (Corsi For / (Corsi For + Corsi Against)) * 100
Evaluating Corsi
- Over 55%: Total dominance. When this player is on the ice, their team controls the puck and the game.
- 50%: Perfectly even play.
- Under 45%: The player is trapped in their own defensive zone, constantly defending against offensive pressure.