I’ve been doing some writing for broader audiences lately, which has led to me thinking a lot about how to convey ideas to people who aren’t five years or more into thinking about hockey in terms of numbers. I think that I’ve come up with a pretty succinct way of expressing why Corsi% matters that I figured I’d lay out here as a resource for other people writing this stuff who run into the same objections.
First, a hockey team’s objective is to outscore its opposition. Players want to be on the ice for more goals for than they are goals against. In other words, they want the percentage of the goals that their team scores when they’re on the ice to be above 50%, something known as GF%. The greater your Corsi%, the greater the chance that that’s true for you.
The cunning observer will note that if 80.9% of players with a Corsi% above 55% saw their teams score more than 50% of the goals when they’re on the ice, that means that 19.1% of players did not. This leads to my second point. Except at the extremes, if your GF% greatly exceeds or falls below your Corsi% in one year, you can expect it to match your Corsi% the following year.
I can illustrate this by taking the 2330 players who played at least 500 minutes in consecutive seasons between 2007-14 and creating groups based on the difference between their GF% and their Corsi%. So, for example, in 2012-13 Erik Gudbranson had a GF% of 23.8%. His Corsi% was 47.9%. That’s a difference of -24.1%. I can create groups of players who had similar seasons in terms of the difference between their GF% and Corsi% and then look at the difference between their GF% and their Corsi% in the following season.
So, for example, we can see that the group of players who had a GF% of at least 10 points worse than their Corsi% in Year One averaged 12.9% worse in Year One and saw that gap fall to 2.6% in Year Two. The guys at the other end of the scale saw the gap fall from 12.9% in Year One (ie. a GF% of 62.9% and a Corsi% of 50%) to just 1.4% in Year Two (ie. a GF% of 51.4% and a Corsi% of 50%). Every other group was within a point in the second year.
Once you accept this concept, it changes a lot in terms of how you think about hockey. It means that when we’re talking about players and teams and how good they are, we should be very, very leery of players/teams whose Corsi% deviates significantly from their GF%. This isn’t to say that some players can’t create an enduring edge here – Sidney Crosby had a 5v5 GF% of 62% from 2007-13 on a Corsi% of 53% – but this is not the norm. For most players, their Corsi% and GF% are going to be very tightly intertwined in the long run. In the short run, GF% is a liar.Email Tyler Dellow at email@example.com