The Bozak-Corsi Dilemma

(Note: This is a cross post with MapleLeafsHotStove.com. You can find the original article here. I don’t normally cross post but this is relevant to Hockey Analytics as a whole, not mostly to Maple Leaf fans.)

A significant portion of modern hockey analytics revolves around Corsi (or SAT% as defined by the NHL), which is really nothing more than looking at which team takes more shot attempts. If you can out shoot your opponent, the theory is that it goes a long way to driving success in terms of out scoring your opponent and ultimately winning games. There is a lot of evidence to support the case that Corsi is a major component of on-ice success. While I believe many people put too much weight on Corsi statistics, I do accept that it is a major component of success.

Over the past few weeks, I have looked at the Leafs performance this season under Randy Carlyle and under Peter Horachek. First I looked at how zone start usage has change from Carlyle to Horachek and the impact of those changes on Corsi. Last week, I looked at a WOWY analysis of Tyler Bozak and David Booth to see if change in linemates from Carlyle to Horachek accounted for the changes in results. The conclusion from these posts is that a significant portion of the Leafs’ improved Corsi statistics is driven by the Leafs top line, and that outside of the top line not a lot has changed with respect to their Corsi statistics. To highlight the improvement in the Leafs top line, here are their 5v5tied CF% statistics under Carlyle and under Horachek.

Bozak CF% Kessel CF% JVR CF%
under Carlyle 38.4 41.0 39.0
under Horachek 53.6 52.0 55.0
Difference 15.2 11.0 16.0

Under Carlyle, the trio of Bozak, Kessel and JVR were pretty close to a league-worst Corsi line, with Bozak being the worst of the three. Under Horachek, they are well above the break even 50.0% line and have put up pretty good Corsi percentages. As far as Corsi is concerned, this trio went from downright awful to well above average. All it took was, I presume, a playing style change demanded by a new coach.

For several years it has been believed that Corsi is an important tool in evaluating players. It was a major component of what has driven the analytics community to conclude that Bozak is a poor hockey player. The evidence above suggests that a simple playing style change can drive Corsi from downright terrible to pretty good. This leads to a bit of a dilemma within hockey analytics, which I will call the Bozak-Corsi dilemma, with two serious questions that need to be answered:

  1. Is Bozak now a pretty good player?
  2. More importantly, if a player (or a forward line) can dramatically alter their Corsi overnight seemingly solely through changing playing style (driven by a coaching change), it must be concluded that Corsi is not primarily driven by individual player talent.

The first point will provide some angst within the Leafs fan base, but from my perspective the answer is no because of his (goal) WOWY’s, Points/60, IPP, etc. are also pretty weak, although maybe he isn’t as bad as previously thought if he plays an optimal playing style.

The second point is critically important, though, because it basically implies that Corsi has significantly less value (maybe little or no value) in individual player evaluation than previously thought, which should send ripples throughout the hockey analytics community. If Corsi is largely driven by playing style, one must conclude it isn’t an individual skill? It isn’t something I’d conclude based on three players, but it definitely makes you think about it more.