Cult of Hockey: Corsi stat may not be a fair evaluation of Schultz
MacTavish considers Oilers defenceman part of Oilers’ core group
EDMONTON - The hottest advanced stat in hockey? When both TSN’s Bob McKenzie and CHED’s Bob Stauffer are dropping reference to Corsi, you know that the stat has gone supernova.
Corsi is much like the NHL’s official goals plus-minus number, but while plus-minus is based on goals for and against, Corsi is based on shots at net for and against.
Corsi is most commonly expressed as a percentage, so a player who has been on the ice for 400 shots for and 600 shots against will have a Corsi number of 40 per cent. Shots at net include missed shots and blocked shots.
How much weight can be put on this stat? That’s the key issue. It’s of particular importance right now to the Edmonton Oilers, who are in contract negotiations with young defenceman Justin Schultz.
Oilers general manager Craig MacTavish has talked about Schultz being one of Edmonton’s core players, along with Taylor Hall, Jordan Eberle and Ryan Nugent-Hopkins. That’s $6 million per year company.
But MacTavish’s fine appraisal of the 24-year-old defenceman comes as Schultz’s Corsi number is one of the worst on the team, just 42.9 per cent. It was worse than plodding defenceman Mark Fraser, who was at 43.1.
Schultz’s poor Corsi is leading some bloggers and analysts to criticize his game and include his name in trade speculation. My take leans far more in the direction of what MacTavish sees in the player. I don’t entirely buy what shots at net/plus-minus says, no matter how trendy Corsi is.
You see, Corsi is like a tempting hotdog. It looks good, smells good, might even taste good, but it can you leave you with a stomach ache. If you actually dig into what ingredients are in it, you might start to question your consumption.
Let’s start with the fact that Schultz’s Corsi isn’t his number at all. Instead, it was earned by all 34 position players that he shared the ice with at even strength. His Corsi is made up of 5,156 minutes of their play and 1,289 minutes of his own play. For example, Andrew Ference was on the ice with him for 464 minutes, Hall 461, Nugent-Hopkins 453, Eberle 423, David Perron 399, and Sam Gagner 322.
Schultz’s Corsi derives from the good and bad plays that all of the players made together, and the others cumulatively had much more impact on that 42.9 per cent Corsi number than Schultz did.
Most importantly, Schultz was assigned a minus mark every time a teammate made a mistake that led to a shot at net against, even if he made no mistake on the play himself. How often did he get such marks, both positive and negative, that he didn’t deserve?
I have not studied the video to assign actual plus-minus marks to Schultz on the 1,036 shots at net for and 1,381 shots against while he was on the ice at even strength in 2013-14. But from my own study of scoring chances, I can draw some conclusions. Schultz contributed to 208 even-strength scoring chances last season and made mistakes on 201 scoring chances against. Overall, his ratio of chances for to chances against was better than every other Oilers defenceman.
If these same rates hold true for Corsi, Schultz helped Edmonton create about 775 shots at net and made mistakes on 769 shots against. In total, he had some role in 1,544 of the 2,417 shots at net while he was on the ice, but no involvement in 873 shots, so about 36 per cent of his Corsi total was false negatives or false positives. That’s a lot of grey area, a lot of uncertainty.
Whatever happened on those hundreds and hundreds of shots at net where Schultz wasn’t significantly involved in the play can’t reasonably be pinned on him. That puts into question the validity of his overall Corsi number. He could easily be much better than it looks.
None of this is to say that Schultz was anything close to an all-star last season. He was especially weak in the corners and off the rush, where bigger, faster forwards roared past him to the Oilers’ net all too often.
But based on his decent scoring chances numbers, it’s clear that the smart and skilled Schultz has had a promising start to his NHL career. MacTavish’s assessment of Schultz as a key part of the Oilers’ core group is fair enough.
Journal columnist David Staples writes regularly for the Cult of Hockey analytics blog.
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