Jim Corsi: The man behind Corsi numbers
The hockey world is ages behind “Moneyball” philosophy and Sabermetrics, just to name a few buzzwords associated with the innovative stats championed by baseball writers and stats gurus for years. That isn’t to say that hockey bloggers/writers/fans/front office people are completely turning a blind eye to analysis that goes deeper than goals, assists and plus/minuses, though. One of the most well-known “new” stats is the Corsi Number, named after Buffalo Sabres goalie coach Jim Corsi (he’s the guy in that photo who’s either short ... or maybe standing next to Tyler Myers). To explain the stat (a bit too) simply: it’s basically an alternative to plus/minus in which shots replace goals. Here’s a more detailed description from David Staples’ great story about Corsi (the man and the stat).
Corsi is a plus/minus stat that measures shots directed at net. For example, if the Edmonton Oilers direct 30 shots at even strength at the Calgary Flames’ net in a game, while the Flames direct 45 shots at the Oilers’ net, the Oilers have a Corsi of -15 for the game. People who put a lot of faith in Corsi plus/minus numbers argue that you even if a team loses a stretch of games, but if that team has a strong Corsi plus/minus in each game, it’s an indication that the team really isn’t so bad, that it’s moving the puck to the right end of the ice, that it has territorial dominance, and that the goals will soon come.
This team Corsi number is also broken down and applied to individual players. For example, if Sheldon Souray is out on the ice for 10 shots directed at the Flames’ net, while the Flames direct 15 shots at the Oilers’ net while Souray is on the ice, he’s said to have a Corsi plus/minus of -5.
The traditional way of measuring a goalie’s work load was to look at the number of shots against the goalie faced in a game. But having played goalie himself, Corsi didn’t trust that basic indicator so much. “When I played, I’d finish a game with about 25 shots on goal and I was really tired. I was really very, very tired,” he says.
“When you look at 20 or 25 shots, you say, Well, it’s a light game.’ But if you look at a game, an (attacker) comes down the right side, he’s got the puck, and he turns up the boards, towards the wall, the goalie is not standing on his heels going, ‘Oh, you know, whatever.’ He’s going to bear down, tense up, prepare that this is going to end up as a shot. Now as (the attacker) turns, he (the goalie) has still got to be engaged and wait for the next play.
“You get a 25-shot game, but with all the blocked shots and the increase in blocked shots and the tightness of the defensive zone coverage, the activity of the goalie could almost be like 75 to 100 actions at the net.”
To get a better sense of how much work his goalies faced, Corsi started to use the NHL’s data that tracks missed shots, blocked shots and shots on goal. He added all these together to come up with a goalie’s so-called Corsi number for a night of work. “It’s really just a number that I’ve used for goaltenders to hone in on their fitness and how much activity is actually going on with respect to goaltending.”