It must be 2010; I’m watching Don Cherry talk about Corsi numbers on TV [fast-forward to 5:00 in the video above]. Granted, he’s denouncing them, but a) he has a couple of good points, and b) that’s what old guys do when they’re confronted with statistical innovations. Read your Kuhn.
Ron Maclean didn’t do a very good job of explaining the Corsi stat (yes, it was invented by Jim Corsi), and he picked a slightly inopportune occasion to bring it up with Grapes sitting next to him. As this Globe & Mail primer explains, Corsis are essentially a more powerful extension of the plus-minus stat you see in the newspaper; they count not only the goals for and against while the player is on the ice at even strength, but all shots directed at the net either way (goals, shots on goal, missed shots, and blocked shots).
Everybody knows plus-minus isn’t a very robust or accurate way of measuring a player’s contribution, and Corsi numbers mitigate some of the disadvantages of only counting goals. You’re counting a lot more events per game—scoring chances, loosely speaking–which gives you more statistical power and leaves luck and contextual factors with less relative influence on the stat. You’re also factoring out the quality of the goaltending behind (and in front of) a skater.
That doesn’t mean Corsis are a perfect means of understanding or isolating a player’s contribution. Shifts in hockey aren’t like a batting order, in which everyone must take his turn. Some players are out there with inferior teammates, some players are shielded from the toughest competition, and some players provide value just by chewing up a lot of minutes. Context is important, and in hockey we may never be able to correct advanced stats for context as well as we can for hitters in baseball. (That’s why stats in hockey aren’t very advanced. We’ve really only just gotten around to expressing events as rates in the simplest possible way. The guy who did this for baseball was born 186 years ago.)
The biggest easily-measurable influence on Corsi numbers—easily measurable thanks to the work of Gabe Desjardins—is where a player tends to start his shifts on faceoffs. A guy who is rolled out for a lot of defensive draws is going to have a worse Corsi rating through no fault of his own—indeed, he is penalized for being trusted by his coach. In that sense, Ryan Johnson was a bad choice for Maclean to pick on, and Don Cherry’s outburst of skepticism was entirely appropriate. Desjardins’ site also tracks “zone starts”, so we know that Johnson, who has the league’s worst Corsi rating, is one of the league’s most disadvantaged regular skaters zone-wise. Through the games of March 28 he’s been sent out for only 78 offensive-zone faceoffs but 165 in his own end. Which is why he’s near the very bottom of this list.
Like Desjardins himself, I am less impressed by the subtly different argument actually made by Cherry—that it’s unfair to penalize Johnson for blocked-shots-against that he himself has blocked. Insofar as Corsi numbers are measuring any ability, it’s the ability to not have to block shots in the first place—to help your team promote the puck out of your end and into the enemy’s defensive zone. The counting of blocked shots has a problem similar to the counting of double plays turned by a team in baseball; they correlate negatively, if at all, with the winning of games. An individual blocked shot might have a positive value—though even that’s certainly not true in every case—and you want players who are willing to block them. But it’s better not to give up lots of opportunities for blocked shots.
And, hell, it’s better still not to be a low-talent, high-grit player who has to block them to keep a job. Even Don Cherry knows that.
Sunday, March 28, 2010