There was a lot of debate regarding advanced analytics this past offseason. The debate was filled with passionate supporters on one side and adversaries who vehemently rejected the statistic as relevant on the other. This blog is not an extension of that debate and does not challenge the relevance or significance of advanced analytics such as Corsi; however, it does challenge the theory that Corsi is an accurate proxy for possession.
People often interpreted Corsi as a statistic that demonstrates which teams or players drive possession and it appears they may be wrong.
Scott Cullen, the analytics guy for TSN and a strong believer in Corsi, tweeted today that based on a report, Colorado’s timed offensive zone possession for the 2013-14 season ranked in the top 5 of the NHL along with Chicago, Los Angeles, Boston, and St. Louis. Despite their high ranking, they ranked 27th in the league in Corsi during the 2013-14 season. This directly contradicts the idea that Corsi is a possession proxy.
To be clear, Scott Cullen cited Mike Kelly, a host on the NHL Network, as the source of the report. Kelly hasn’t given any further information regarding the report, but for the sake of this blog, we’re going to assume his information is accurate (advanced analytics supporters love assumptions).
Corsi is calculated by taking total shot attempts for and dividing them by total shot attempts against. The logic is that if a team possesses the puck more, it’s safe to assume they likely take more shots. It started as a team metric but was soon applied to individual players as well. It’s easy to see why that concept makes sense; however, at the end of the day, it’s simply an assumption. Additionally, Corsi uses imperfect and inconsistent statistics in its calculations: shots are recorded differently in every NHL arena. Again, pro-analytics supporters argue that it’s safe to assume that over the course of an 82 game season, everything balances out. Another assumption.
It’s difficult to argue that high Corsi numbers don’t directly correlate to wins. There has been a lot of research done in this regard and generally speaking, successful teams rank towards the top of the NHL in Corsi. Of course, there are a few exceptions to the rule (New Jersey ranks high but missed the playoffs, Colorado ranks low but made the playoffs) but regardless, high Corsi means wins. And it makes sense that if you outshoot your opponent consistently, then you have a higher chance of winning. But if Kelly’s information that Cullen cited is correct, it means that Corsi’s correlation to possession is greatly exaggerated and potentially non-existent.