On Saturday night's edition of Hockey Night In Canada, colorful (or colourful) commentator Don Cherry blasted the idea of the Corsi number, an advanced metric created by Buffalo Sabres goalie coach Jim Corsi. The Corsi number measures how many shots are directed at your net in comparison to how many are directed at your opponent's net while you are on the ice.
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This shows how stupid the Corsi thing is. Here's a guy that every coach would want. This is how stupid guys come up with -- trying to earn a living -- they come up with dumb... . They said this guy is the worst hockey player in the league. This guy is unbelievable and they call him the worst, from that dumb thing that you did.
And he's the worst player? I'll tell you one thing. Vancouver loves this guy, he is unbelievable, and that dumb-dumb system you're talking about -- I would love to have Ryan Johnson on my team, and every coach would have him on too.
Gabriel Desjardins of SB Nation's advanced stats blog, Behind The Net, fired back at Cherry this morning, saying that among other things, Cherry just doesn't understand the importance of the Corsi number.
At any rate, one of the things I stress about Corsi is that it needs to be understood in context. Yes, Ryan Johnson has the worst Corsi among regular players in the NHL - but at 5-on-5, he has also taken 67.9% of his faceoffs in Vancouver's defensive zone. Johnson plays primarily on the 4th line and spends a lot of time dragging Darcy Hordichuk and Rick Rypien up-and-down the ice, so we'd expect his numbers to suffer for lack of better linemates. That's why at -2, Johnson has one of the 'worst' plus/minuses on his team.
One other issue that Don Cherry brings up is that shots that Ryan Johnson personally blocks are counted against him. And why wouldn't they be? When you block a shot, it means the other team had puck possession in your zone while you were on the ice. That's not a positive thing. Obviously when we evaluate Johnson as an individual, we would take his shot-blocking ability into account. But if Darcy Hordichuk was on the ice with him and allowed the other team to control the puck - if we're trying to measure puck possession, we shouldn't give Hordichuk credit for Johnson's skill.
It's non-trivial to determine a player's value. But Corsi does a good job of determining where the puck was when a player is on the ice. It's heavily-driven by whether you start out with a defensive or offensive draw, and obviously it assigns group credit for individual skills. But so do most other statistics that we don't sneer at - play with Mario Lemieux and you'll score a bunch of goals; it doesn't invalidate goal-scoring. I have a feeling that if Don Cherry had coached an NHL team in the last 30 years, he might not be so dismissive of Jim Corsi.