Editor's Note: Over the summer we're going to try to put together some primers on some of the statistical measures that fall under the "advanced statistics" umbrella in order to help lay the groundwork for a lot of the analytical discussion that takes places on the site and throughout a lot of the Bettmanosphere (trademark pending). This is a great chance to ask any questions, seek any clarifications, and share any links that can help increase everyone's undertanding (ie no "advanced stats are stupid" links).
What does the stat actually describe?
SHOT ATTEMPT DIFFERENTIAL
Where did the weird name come from?
Named after Buffalo Sabres goaltending coach Jim Corsi
What it means and why it's useful
A player's Corsi Number is just the sum of all shots directed by a team towards the opposition net (shots on goal, missed shots, and blocked shots), minus the sum of all shots directed by the opposition towards your team's net. For an individual player the Corsi number is just the value determined while they are on the ice.
Corsi = Shot Attempts FOR - Shot Attempts AGAINST
Corsi is calculated game by game - for example here is an individual game sheet from the last game of the season between Toronto and Montreal courtesy of TimeOnIce.com. On the sheet you can see the players in the game, and all of the goals, saved shots on goal, missed shots on goal, and blocked shots on goal FOR and AGAINST while each player was on the ice. Corsi is just all of the attempts FOR minus all of the attempts against.
What it can tell us
We use Corsi as a proxy for puck possession. Logically, the team that has the puck more is going to make more shot attempts. Generally this has been shown to be the case in studies of actual puck possession time, scoring chances, and general shot attempts. The correlation is very high amongst these variables.
So basically, what this means is - players with high corsi numbers are on the ice for a higher number of shot attempts for their team than the other team. That is to say, their team has the puck more when they're on the ice.
While a certain player may not see a positive corsi result in an individual game, their cumulative corsi over time should indicate their overall possession. This long term seasonal corsi value is shown in a couple of different ways.
Corsi ON = (Total Corsi) x (60 mins) ÷ (The player's total ice time)
Corsi % = the percentage of Corsi events a player's team has while they are on the ice
Corsi REL is another statistic that is meaningful here, as it indicates how solid a player's possession statistics are relative to those of their team-mates. Similarly to how we tabulate Corsi ON, we can also tabulate Corsi OFF, which is the team's overall Corsi number while a player is OFF the ice. Corsi REL is just the difference between these two values.
Corsi REL = Corsi ON - Corsi OFF
Corsi ON, Corsi OFF, and Corsi REL are all tabulated using the same per 60 minute basis on behindthenet.ca and can be found under the Statistics pull down menu, on the "player breakdown" page. The lists can be sorted by team, position, minimum gp in a season, etc.
Here is the Corsi page for the Leafs from this past season. The players are ranked by Corsi REL - and this is a very solid indication of who on the team does the best job of maintaining possession of the puck (let this give you hope Nazem Kadri fans! in 21 gp last year he ranked 2nd on the team behind Mikhail Grabovski).
Corsi ON and Corsi REL are both used to help determine quality of competition also, by averaging out the puck possession skills of the players that a skater faced during the course of the season. I will explain this in more detail in a subsequent posting though. For now, just work on wrapping your head around the idea of shot differential equating to possession.
By looking at this shot chart, you can see that the Leafs were outworked for the first 2 periods of the game, and didn't really wake up until the start of the 3rd, by the time they were already down 2-0. The early possession didn't really favour either team, but Montreal got 2 goals despite that fact. They started to press their advantage heavily in
All of these impressions add up to a solid concept of the fact that SHOTS are an indication of WHICH TEAM IS DRIVING PLAY. Whoever is shooting more, has the puck more, and the team with the puck more tends to score more often. This is how hockey games are won. It might not overcome bad luck every night, but over the course of a full season, it's pretty important.
Teams that win the possession battle tend to be better off. If you want a clearer example of that, take a look at the Corsi ON ratings of the Leafs players here. Then compare them to the Corsi ON ratings of the LA Kings, the St Louis Blues, and the Detroit Red Wings.