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Corsi and Fenwick Primer

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Megan Kim

on 11 November 2016

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Transcript of Corsi and Fenwick Primer

What are Corsi and Fenwick?
Corsi and Fenwick are your two run-of-the-mill shot metrics. They’re not possession metrics in and of themselves, but they operate as a proxy for possession. (We don’t keep track of possession time in hockey like we do in soccer.) The logic here is that if you’re shooting the puck, you possess it, so the more you shoot the puck, the more you’ve had the puck.
✳✱*
CORSI
Corsi is found by adding up shots on goal (that’s the number you see on the shot clock during a game), missed shots, and blocked shots.

All of the above are considered "Corsi events."
FENWICK
Fenwick is very, very similar to Corsi -- it counts shots on goal and missed shots. (Fenwick excludes blocked shots because it’s totally possible that blocking shots is a skill rather than a random event -- shoutout to Kris Russell, I guess.)
Corsi & Fenwick
I promise they're useful!
Calculating
Corsi & Fenwick

Basically... it's just a bunch of counting.

The way we count Corsi and Fenwick is “For” and “Against.” For example...
how do we find Corsi and Fenwick?
It's easy! Seriously!
(to calculate fenwick)
Okay, so say this made-up game is between Connor’s Oilers and… the Canucks. Sorry, Canucks fans. It’s not because I don’t like you, it’s because this is just the easy thing to do.

Let’s say that while the very handsome Erik Gudbranson. is on the ice, the Canucks get 1 shot on goal and miss the net 2 times. (Remember, no blocked shots for Fenwick!) Meanwhile, the Oilers get 3 shots on goal and 5 missed shots.

Using the same process as when calculating Corsi, Gudbranson’s Fenwick For is 3, his Fenwick against is 8, and he’s a -5 Fenwick on the evening. That turns into a brutal 27% FF%. (Anything under 50% is less than optimal, anything under 30% is pretty abysmal.)
note: you don't have to count these by hand. more on that later.
(to calculate corsi)
Connor McDavid (in an entirely made up game) is on the ice for 4 EDM shots on goal, 4 EDM missed shots, and 3 EDM shots that were blocked by the other team. That’s a total of 11 shots. During his time on the ice, the other team has 1 shot on goal, 2 missed shots, and 1 blocked shot. (Kris Russell blocked the shot, he’s good at that.)

That means Connor’s Corsi For (CF) is 11, his Corsi Against (CA) is 4, and he’d be a +7 Corsi (11 shots for - 4 shots against = 7) on the night. And because percentages are nicer to work with, we turn that into a 73% CF% by dividing his CF (11) by total Corsi events (11 shots + 4 shots = 15 overall events).
BUT WAIT...
These concepts work in other contexts too – you can check a player’s Corsi and Fenwick over stretches of time instead of just individual games, and you can also check overall team Corsi and Fenwick for individual games and stretches of time!
and even if this is
your first time looking
into these stats, i promise
they're really intuitive.
NOW YOU KNOW
Corsi and Fenwick are basically just weird words that mean "counting shot attempts in order to estimate possession."
So... Where can I find this nifty information so I do not have to count shot attempts manually?
Glad you asked!! There are some really cool resources for this stuff.

First and foremost is the wonderful Corsica, which you can find at
http://corsica.hockey
Hands down the best stats site around right now, with skater, team, and goalie stats. There are also game-by-game stats and other cool features. The home page looks like this.
(individual skater stats)
(game stats)
(goalie stats)
(team stats)
(skater combination stats)
Another terrific place to go for data is
http://hockeystats.ca. This is honestly my go-to for in-game data, especially since you can check current games. This is where people get those handy possession charts that look like this:
(there are a BUNCH of different charts you can generate here -- I'm also a big fan of the shot attempt location spray chart!)
These charts are nice because you can see which team possesses the puck more at a given time during the game. In this example, Arizona definitely carried the play.
And one more site that I check on the reg --
http://naturalstattrick.com. The interface isn't as clean and pretty as the other sites, but the player combination data is really thorough and really interesting.

For instance, the chart below shows Shea Weber's teammates' numbers with and without him. It's too early in the season to really take any of this seriously, but I for one am keeping an eye on Alexei Emelin's numbers with Weber.
Also, http://hockeyviz.com is one of my favorite resources out there. There are really cool vizualizations on this site -- some look intimidating, but every viz has a corresponding "how to read" entry on the site. (Micah Blake McCurdy, who creates the vizualisations and runs the site, doesn't use the term "Corsi," choosing instead to simply refer to Corsi events as "shots." Which, of course, is exactly what they are.)
IN CONCLUSION: Are Corsi and/or Fenwick infallible? The One True
Advanced
Stat?
What? No, of course not. No one stat is ever going to give you a complete picture of a player/team/game/etc. And Corsi/Fenwick certainly aren't the only ways to evaluate the performance of a given team or player. They may not even be the best way, but they're a darned sight better than something like, say, +/-.

But we've seen that a good Corsi tends to correlate with success and like, Stanley Cups.

Like everything in life, use Corsi with moderation. It isn't the be-all, end-all of hockey stats, but it certainly is useful.
Last, but definitely not least.....
Hockey Twitter is a big and sometimes scary place. Hockey Analytics Twitter gets a bad rap sometimes, but here are just a few (of the many) incredibly kind and smart people who are always willing to share and explain their immensely valuable work to get you started.


Micah Blake McCurdy (@IneffectiveMath)
Sean Tierney (@ChartingHockey)
Carolyn Wilke (@Classlicity)
Ryan Stimson (@RK_Stimp)
Corey Sznajder (@ShutdownLine)
Prashanth Iyer (@iyer_prashanth)
Charlie O'Connor (@BSH_Charlie)
Dimitri Filipovic (@DimFilipovic)
*Several of the people on this list are at the forefront of the tracking movement, which deals with passes, zone exits/entries, and other neat things. But this presentation was getting too long, so we'll get to that next time. ;)
Well... We've reached the end of this crash course on Corsi and Fenwick.

I hope it was helpful to any folks getting started with this whole hockey stats thing, and thanks for following along!

Catch me on Twitter (@meggo1532) if you need clarification/ever want to chat about hockey!
this dude changed his handle
like literally today. thx a lot, sean.
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