The Corsi number was named by Tim Barnes, a financial analyst from Chicago working under the pseudonym Vic Ferrari. He had heard former Buffalo Sabresgeneral managerDarcy Regier talking about shot differential on the radio, and then proceeded to develop a formula to accurately display shot differential. Ferrari originally wanted to name it the Regier number, but he didn't think it sounded right. He then considered calling it the Ruff number after former Buffalo Sabres head coachLindy Ruff but he didn't think that was appropriate either. Ferrari ended up searching Buffalo Sabres staff, found a picture of Jim Corsi, and chose his name because he liked Corsi's mustache.
Formulae
Corsi For = Shot attempts for at even strength: Shots + Blocks + Misses
Corsi Against = Shot attempts against at even strength: Shots + Blocks + Misses
Relative Corsi per 60 Minutes at Even Strength = CF/60 - CFoff/60 = On-Ice Corsi For / 60 Minutes - Off-Ice Corsi For / 60 Minutes
Relevance
Because shot attempts are often taken in the offensive zone, Corsi numbers provide an indication of the time a team spends in the offensive zone, versus time spent in their defensive zone. Positive Corsi numbers tells you that the team spends more time in the offensive zone than the defensive zone, while negative Corsi numbers tells you the team spends more time in the defensive zone than offensive zone. Possession is a good stat to take into account because the team with the higher possession numbers at the end of the game generally wins. Corsi can be broken down into four categories: Corsi Ahead, Corsi Even, Corsi Close, and Corsi behind. In order, the categories refer to what a team or player's Corsi number is when they are ahead in the game, when the game is tied, when the score of the game only differs by one goal no matter who is ahead, and when the score is behind, in that order. Each of these stats will tell a coach whom he should be putting out on the ice depending on the score of the game.
Criticisms
While Corsi may provide a more accurate evaluation of players' contributions to a winning effort than plus-minus, it does have its own criticisms. For example, a player is on the ice for 30 shots on net and 20 shots against. This player is a defensive defenseman, and in this situation he only helped create 5 shots on net but his mistakes led to 15 shots against. This tells us his real impact on the game is -10 Corsi, but on the game sheet at the end of the game his Corsi number will be +10. This is because he was playing with better players around him and that boosted his Corsi number. Essentially, a good player playing consistently with bad players will have a lower Corsi number, while a good player playing with great players will get a boost. For this reason, additional advanced metrics, for instance using raw Corsi values adjusted against frequent on-ice teammates', can provide a more accurate assessment of individual players' contributions.
