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The corsi statistic is an advanced statistic used in the National Hockey League. Corsi is the total shots at the net for and against at even strength. It’s often expressed as either a differential, like plus/minus, or a percentage. The corsi statistic is a proxy measure of offensive zone possession.

History
The corsi statistic is named after Jim Corsi, but he is not the one to create it. An outside-the-box thinker, Corsi was trying to measure just how busy his goalies were in a game. He didn’t believe the simple shots-against total — usually around 30 a game — was totally reflective of just how busy a goalie was. And since it’s up to the goalie coach to ensure the goalie is in shape, it was up to Corsi to find out how much workout time goalies needed between games. Corsi stats that “Another fellow took my stats and started applying it to players to find out what their contribution was overall, on both sides of the puck, and voila, the Corsi stat came out.”

The corsi statistic was named in 2004 by Tim Barnes, a engineer turned financial analysts from Canada but currently resides in Chicago. Barnes help develop, popularize, and spread the stat. Barnes did this by blogging anonymously - and more famously - under the pseudonym, Vic Ferrari. Barnes, with the help of other blogger, Gabriel Desjardins, ideas took root when many NHL fans were unsatisfied with the plus-minus stat, in large part because they believed goals were akin to random events. There are more shots and shot attempts in a game than goals and shots on goal, meaning the larger sample size of Corsi events is more reflective of a player’s performance than whether a player is on the ice for an unlucky bounce. Barnes turns out to be, the other fellow.

Relevance
Corsi is a proxy measure for offensive zone possession. Players and teams with positive Corsi rates tend to spend more time in the offensive zone at five-on-five, something that’s predictive of success over the long-term. In general, the higher the corsi differential or ratio, the more dominant the team or player. At the same time, the blogosphere was trying to find out what teams were best at puck possession. The NHL dropped time of possession as a stat in 2002. So bloggers turned to Corsi as its proxy, figuring if one team shot the puck more than the other team, it meant one team controlled the puck more than the other team. By measuring the various shots in 5-on-5 situations — like plus-minus — it puts all players on a level playing field. The stars don’t get the advantage that power plays give them. The grinders aren’t hurt by their time killing penalties. They can be measured with various linemates and against strong or weak opposition.

Starting with Corsi, there are two kinds of Events: a Corsi For Event and a Corsi Against Event. Again, despite the names, those are quite simple: a Corsi For Event is a shot attempt by your team, a Corsi Against Event is a shot attempt by your opponent. Corsi Events include all shot attempts, regardless of whether they're saved, blocked, off-target, or scored.