Corsi Explained

well well well, if it isn't "Mr. Corsi" himself. - Doug Pensinger

Kings color analyst Jim Fox talked about Corsi a lot last night. What exactly is it and why should you care?

If you watched the Kings broadcast last night, you may have heard color commentator Jim Fox talking about a lesser known hockey statistic known as Corsi. Fox did a great job of breaking down Corsi in a concise and accessible manner, but it is probably a little difficult to explain the nuances of it and why it’s important during a broadcast. So this post is an effort to fill in some of those gaps if you’re interested.

“What is Corsi and why should I care?”

First, a little bit of background. The stat that has now come to be known as “Corsi” was initially developed by current and longtime Sabres goaltending coach Jim Corsi. The Sporting News’ Jesse Spector profiled Jim Corsi in an article last year:

“I was trying to measure the amount of work that a goalie does,” Corsi says. “What happened along the way, this fella, an engineer in California … tied it into the work that players do, and he sorted it out in a way that reflects the work that players do. He was kind enough to say it was based on my work originally. Hence, we have this Corsi number.”

Initially, Corsi’s metric was an effort to track how many events his goaltenders saw per game. As Spector’s article points out, a few hockey analysts took that methodology and applied it not just to goaltenders but to skaters and entire teams. What these analysts found was that this metric correlated very highly with zone time and winning. It was eventually dubbed "Corsi" to honor the coach that initially developed it.

Corsi takes into account all shot attempts (shots on goal, shots that missed the net or hit the post, and shots that were blocked by the opposition) at even strength. When a player is on ice at even strength for an attempt at the opposing goal, they get a plus. When they are on ice for an attempt against, they get a minus. It’s as simple as that. As Fox, explained last night it’s basically just plus/minus except it uses shot attempts at even strength instead of goals at all strengths.

You might remember that for a brief time period the NHL tracked zone time in their official game summaries. They stopped doing this after 2002, which was rather unfortunate since zone time is a very important facet of hockey. Well, what Oilers blogger Vic Ferrari found was that Corsi correlates with zone time about as perfectly as one thing can correlate to another.

Also more recently, Maple Leafs blogger Draglikepull did some work tracking zone time by hand and compared it to Corsi and other possession stats. He, like Ferrari, found that Corsi and zone time are extremely closely related.

People have found that possession metrics correlate very strongly to winning hockey games and championships.

So why is any of this important? Well, because people have found that possession metrics also correlate very strongly to winning hockey games and championships. In fact, back in 2011-12, when the Kings were struggling, people who understood these possession stats were predicting that the Kings would do some serious damage. These stats also predicted the hard fall of artificially inflated teams like ‘09-10 Avalanche, ’10-11 Stars, and ’11-12 Wild. They are also now currently predicting that this year’s Maple Leafs are on the verge of a catastrophe.

In fact, possession stats have shown to be more predicative of a team's future performance than wins themselves.

“How do I use it?”

First of all, it’s important to understand that Corsi is just a tool. It’s not a magic number. It is something that is best used given the proper context. You can get the proper context by taking into account things like score effects, zone starts, the quality of completion faced and who an individual player has played with.

It is certainly not the be all and all, but if you’re a regular human being that isn’t able to pour over film of every hockey game played, it can help give you a pretty nice picture of what’s going on if you use it right.

“Where can I find it?”

You can find the actual numbers on the team level at Stats.Hockeyanalysis.com and Extraskater.com. CF% stands for “Corsi For Percentage”. It is just the difference between Corsi events for and against represented as a percentage.

Greater than 50% and a team has been controlling the action more often than not. FF% is “Fenwick For Percentage”. "Fenwick" is just Corsi minus blocked shots. Over smaller samples, it’s better to use Corsi because it is composed of more events. In larger samples, Fenwick gives you a slightly nicer picture.

You can find the individual numbers at the same sites, but I’ll provide you links for the Kings here and here.

Also, Behindthenet.ca is a great resource and the one that started it all.

“What about other hockey stats?”

There are an array of newer hockey stats out there these days. We here at JFTC are tracking things like zone entries which is a new project which attempts to get a better understanding of neutral zone play. There is also a zone exit project which tracks individual success rates for players when attempting to breakout of the defensive zone. We’ll have more on these soon.

In the meantime, you can get a fantastic overview on all the newer stats in this post from our SB Nation colleague Eric Tulsky.

Also, if you have any questions at all please fire away in the comments or feel free to get a hold of us on twitter.

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