My thoughts on Corsi Numbers
I am planning that over the course of the summer and into next season I will get back into analyzing hockey statistics more in depth again. Over the past couple of seasons Corsi numbers have become much more prevalent so I thought I would start off by discussing what they are and my thoughts on them.
Corsi numbers were originally created by former NHL goalie and now Buffalo Sabre goalie coach Jim Corsi. David Staples recently had a good interview with Corsi which goes into his thought process behind developing Corsi numbers. The interview is definitely worth a read but let me summarize.
In his role as the Sabre’s goalie coach, Corsi was attempting to evaluate the work load his goalies had in a game of play and found that simply shots against were not sufficient. The goalie can relax whenever the puck is in the oppositions end, but whenever the play is in his own end he can’t relax, regardless of whether a shot was taken or not. To get a better idea of his goalies workload he summed up shots, missed shots and blocked shots which should give a much better indication of a goalies overall work load. A goalie needs a certain skill level to successfully save the majority of shots on goal, but a goalie also needs a certain fitness level (both mental and physical) to be able to play under a certain workload level within a single game and over the course of an 82 game season and this is why Corsi invented the Corsi numbers.
More recently others in the hockey community have extended Corsi numbers to evaluate a teams ability to control the play of a game (i.e. does a team play more in the oppositions zone vs their own) and evaluate individual players by looking at their Corsi numbers for and against while they are on the ice and comparing that to their teammates Corsi numbers. Most notable are Gabe Desjardins of behindthenet.ca and Gabe and everyone else at the Behind the Net blog but there are others too. Some people, most notably Matt Fenwick of the Battle of Alberta blog only use shots and missed shots and do not include blocked shots as Jim Corsi does resulting in what is typically called Fenwick numbers. When used in this context Corsi and Fenwick numbers are calculated just as +/- is calculated which is to take the shots+missed shots+ blocked shots for his team and subtracting the shots+missed shots+ blocked shots numbers by the opposition while he is on the ice.
One of the benefits that many people believe that Corsi numbers provide is that since Corsi numbers include more events (i.e. shots+missed shots+blocked shots vs just shots or even just goals as in +/-) the statistical analysis will be far more accurate due to the larger ‘sample size.’
So what do I think of all this? I do agree with Jim Corsi that using Corsi numbers as a way to evaluate a goalies workload is probably far more valuable than just using shots on goal. Beyond that, I am pretty sure that Corsi numbers will give a pretty solid indication of a teams control of the play, for whatever that is worth. I say for whatever that is worth because some teams, when they have the lead, will choose to play in a defensive shell allowing a lot of shots from the point, but not giving up all that many high quality, in close, shots or worse yet, shots on rebounds. Corsi numbers when the game is close (tied, or within one goal with significant time to play such that the team with the lead has not yet gone into ‘protect the lead’ mode) may give us a better indication of a teams capability to control the play, when they want to but even that may be flawed. Also, a team with a strong set of forwards but a weak defense and goalie may control the play more than a team with a strong defense and top tier goalie but is that team really any better at winning games?
Much of the same arguments can be made when evaluating players. Defensive minded players are not necessarily on the ice to control the play, they are on the ice to not allow goals against most typically by the oppositions top offensive forwards. As mentioned above, one way to accomplish this is to go into a defensive shell and just not give up any quality scoring chances against. A player can have a sub-par Corsi number, but be doing his job perfectly well.
I do believe that Corsi numbers have a use in evaluating a goalies work load and even in showing which teams are controlling the play, but in my opinion using it anywhere beyond that we are making too many assumptions about how important Corsi numbers are with respect to winning games. Just ask the Washington Capitals how almost completely controlling the play worked for them against Montreal in round two of the playoffs. In the past I have used mostly goals for/against and shot quality (using shot type and distance as a proxy for quality) to evaluate players and while that has its own inherent flaws as well I will most likely continue to do so in the future.