In the last post, I explained how plus/minus should be banished to the fiery depths of hell. So what can we use instead? Jim Corsi to the rescue! You have probably heard of Corsi scores and it’s really not that complex. Jim Corsi, a goalie coach for the Buffalo Sabres, tracked shot attempts to measure the goalie’s workload then the Corsi score was developed.
The formula is not particularly difficult. In its rawest form, it’s shots for minus shots against.
|SF = on ice shots for||SA = on ice shots against|
|MSF = on ice missed shots||MSA = on ice missed shots against|
|BSF = on ice blocked shots||BSA = on ice blocked shots against|
Corsi basically shows that you have more shots on net likely as a result of more possession, which shows you’re good at generating chances.
There is also a similar measure called Fenwick, which is the same as Corsi minus the blocked shots. For example, if Toews is on the ice with 10 shots for and 3 against, he’d have a Corsi score of +7 (10-3). Now, if we say that of those ten shots for, two were blocked, and for the three against, one was blocked, Toews’ Fenwick score would be +6 (8-2). Sometimes the two are expressed as percentages to make them easier to compare.
So why is Corsi useful? Well, it is shown to have a close correlation to scoring chances, puck possession and territorial advantage. It’s a good predictor of success too.
This beautiful chart looks at Fenwick scores from 2007-2013. The rings represent each round of the playoffs and the further a team is from the outer ring is the distance from a playoff berth. Possession percentages are shown as between .400 to .600; the further from .400 the better. +.500 is the magic number as it gives you a 75% chance of making the playoffs; +.550 gives a 25% chance of winning the Stanley Cup. Not bad.
(Outliers do exist of course, but on this chart only one outlier won the cup… the 2009 Pittsburgh Penguins. What happened in that season? They fired their coach and hired Disco Dan who gave them a .599 team Fenwick thus leading them to their cup).
Corsi and Fenwick are good, but remember CONTEXT. The score can be altered by other variables, like their team mates, the competition, or zone starts. So… time for some developments.
Corsi Relative – Measures the difference between the player’s and the team’s when he’s on the bench. This allows us to see what effect the player has on the team. Is the player stuck on a bad team?
Corsi Relative Quality of Teammates – Examines the teammates weighted by time on ice together; Does the player have bad line mates?
Corsi Relative Quality of Competition – Examines opposing players and is weighted by head to head ice time; is the player lining up against the best?
We can also examine zone starts to see if the player is more of a defensive type. This is a usage stat and can be used alongside time on ice to understand how the coach uses the player. There are three types of zone starts: offensive (OZS), defensive (DZS), and neutral (NZS), that show where the player is employed. And time on ice shows us how the coach uses players: even strength (EVTm%), Power Play (PPTm%), and shorthanded (SHTm%).
A final note about Corsi/Fenwick is that they are not perfect. However, scouting is not perfect. Neither is a fan’s opinion. Nothing we have (yet) is perfect, but it is something.