The Corsi Rating and Zone Start

My goal in writing about statistical analysis here on Inside Hockey is actually two-fold:

  1. I want to learn about any developments regarding advanced statistics and/or their applications and then bring that knowledge to you.
  2. I intend to apply new or existing advanced statistics to real NHL players and situations to see how theory measures up to reality.

Using that as my basis, I plan to produce two types of articles. Sometimes I will introduce new (or newly discovered) advanced statistics and debate the merits of their use. The other type of article will be a comparative evaluation of players using some of the advanced statistics that have been introduced.

Today I’d like to introduce an advanced statistic referred to as a “Corsi Rating” or a “Corsi Number.” I originally discovered this on a site called BehindtheNet.com. The Corsi Rating or Number is named after current Buffalo Sabres Goaltending Coach, Jim Corsi. Apparently he devised his rating system as an alternative to the traditional +/- rating. The Sabres use it in its evaluation of players. I can’t tell you how long the Sabres have used this rating but the earliest reference I have found is from November of 2007 on hockeynumbers.blogspot.com.

Basically a Corsi number is figured by calculating the difference between the number of shots DIRECTED toward the offensive goal versus the number of shots DIRECTED toward the defensive goal while in 5-on-5 skating situations and excluding empty net shots. The key word in that definition is “DIRECTED” and it includes all shots that are on net, miss the net or are blocked. I would guess that the rationale behind this stat is that the larger a team’s discrepancy between shots directed for and against, the more that team is controlling the play.

Let’s look at the top five skaters with at least 75 games played from 2008-2009 in terms of highest Corsi Rating per 60 minutes of ice time:

  1. David Moss CGY +23.8
  2. Pavel Datsyuk DET +23
  3. Henrik Zetterberg DET +20.1
  4. Alexander Ovechkin WSH +19
  5. Mikael Samuelsson DET +18.9

The interesting thing I notice is that there are 3 Red Wings players in the top 5. In fact 7 of the top 9 are Red Wings. Not surprisingly the Red Wings led the NHL in Shots on Goal during the regular season with an average of 36.2 per game. Washington was second with 33.5 while Calgary was eighth finishing with an average of 32.2 Shots on Goal per game. While Shots on Goal varies some from the criteria of Shots Directed on Goal used by the Corsi rating it still shows to be a good indicator of which teams have players finishing with higher Corsi Ratings.

The entire Corsi Rating System is more complex than the simple Corsi Number. According to an article posted on the Irreverent Oiler Fans page on vhockey.blogspot.com, it actually goes into more detail than just the number itself. It also includes information about which part of the net the puck is shot into and how many crossbars or goal posts are hit. I don’t know how the Sabres factor that information into an equation or how that helps them evaluate a player but apparently they do and it does.

I guess if I had to judge the usefulness of the Corsi Number my first complaint would be that not all shots (whether they end up on net or not) are created equal. Most hockey analysts prefer scoring chances to shots on goal as a better indicator of which team controlled the play offensively. A shot from the point with no traffic in front of the opposition goalie isn’t a good scoring chance and therefore isn’t an indicator of offensive success.

Another part of this stat that can be misleading is the fact it doesn’t take into account the strength of the opponents or the weaknesses of your teammates when used to evaluate individual players. Granted, the idea is that over the course of a full season, all teams will have faced a fairly equal quality of opponents. This is the reason I used 75 games as the minimum when listing the top Corsi Ratings from last season.

Now we see some flaws as the rating pertains to individual players, what about how the rating applies to teams. At even strength, coaches will try to match lines; especially when there is a faceoff in the defensive zone. Generally a coach will put his best faceoff guy along with his best pair of defensemen to take that draw. No matter what happens here, the “defensive team,” even if winning the draw, will still end up with possession of the puck in their own zone which will make it less likely for them to direct shots at the opposition net. That fact adversely affects their Corsi rating.

As a stand-alone stat, the Corsi number doesn’t strike me as something that NHL General Managers would find much use for when evaluating players. In the contexts described above, it appears that the Corsi Rating falls short. However, is there a way that this stat can be used?

We’ll find out in the next edition of the School of Stats. Thanks for joining me.

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