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Friday, November 14, 2014

How Corsi Stats Impact Mike Johnston's Offensive Zone Line Decisions by @DXTraeger

Zach Sill, O-Zone Master
Superficially, it’s hard to find fault with new head coach Mike Johnston and his team’s torrid 10-3-1 start to the 2014-2015 NHL season.

The Pittsburgh Penguins sit atop the Metropolitan Division with 21 points, and are third in the Eastern Conference overall standings with three games-in-hand against the front-running Tampa Bay Lightning and Montreal Canadians.

The Penguins’ special teams have also been top notch:  Crosby and Malkin have spearheaded a power-play that’s clicking at a near-historic 35.6% clip, while the PK is successful 87.5% of the time while down a man.

Sidney Crosby also leads the NHL in scoring (again), and outside of Olli Maatta’s alarming brush with thyroid cancer and his subsequent surgery, the Penguins have thus far escaped the injury plague that has decimated their regular season on an annual basis.

I acknowledge the swath of Pittsburgh positives because the Penguins- and Mike Johnston- still have some warts that have hindered their ability to capitalize on scoring chances and win more games.

Hockey articles have slowly become inundated with the concepts of “Corsi For” and “Corsi Relative,” terms which are ultimately just a fancy way of determining whether or not a team attempts more shots on goal or gives up more attempted shots on their own goal while any given player or line is on the ice.

The simple idea behind Corsi figures is that a better player and a better team should possess the puck more often than a weaker team, and the ability to chart a player like Sidney Crosby and the number of shot attempts his line generates relative to the number of shots tried by Zach Sill and his linemates gives hockey analysts some tangible comparative data.

Crosby is a fantastic example, and actually leads into an area where Johnston could potentially improve the Penguins’ nightly goal tally.  Because Crosby and Company are so dominant at possession and basically dictate play regardless of where they are on the ice, Johnston understands that he can put his captain, along with Hornqvist and Kunitz, out for a faceoff near Marc-Andre Fleury, and odds are, the three of them will manage to not only get into the attacking end of the ice, but generate shot attempts (giving the Penguins an opportunity to score).

Based on that same concept, the aforementioned Zach Sill and his most recent linemates, Craig Adams and Nick Spaling, do not have particularly good Corsi For statistics (after 19 games, their respective “Corsi On” numbers are -4.43, -4.40, -4.22), and these negative possession numbers suggest that the three of them struggle to maintain possession of the puck.

Given that Sill’s line has problems with puck possession, if Johnston still starts them in their own defending end of the ice, Corsi data gives evidence that Sill, Adams, and Spaling will concede shot attempts, potentially endangering the Penguins to give up a goal.  Johnston’s reaction has been to start the fourth line in the attacking zone, as far away from the Penguins’ own net so as to diminish the negative effects of those players on the ice.

If that assessment of Adams/Sill/Spaling reads as damning, well, it is- but strictly from a Corsi-perspective.  As fans, we already knew that Adams and Sill weren't exactly going to turn into 20 goal scorers, so their lack-of-shot-attempt contributions to the team have to be weighed accordingly, and facets such as penalty killing (where Adams excels) and in the much-maligned notion of “eating up minutes” so the big horses of Crosby and Malkin can recharge should be considered when grading out their play.

Still, Johnston’s active decision to place the fourth line on the ice for offensive zone face-offs is slightly curious because nothing mandates that he use his fourth line of Sill/Adams/Spaling in that situation.  For years, the Penguins have made hay by having Crosby and Malkin win draws in the attacking zone, and immediately generating high percentage shots on net with someone (usually Kunitz) screening the net minder.

In essence, Johnston is eschewing the opportunity to have his best offensive players and their lines take offensive draws to execute high percentage set plays that give the Penguins a higher-than-average chance at scoring a goal in favor of having his worst possession players start far away from their own net.

The question of “offense” versus “defense” in determining lines for an offensive zone is a decision sometimes dictated by circumstance: if Crosby or Malkin’s lines had just come off of the ice following an extended shift, fatigue obviously dictates whatever assignment Johnston runs with.  Similarly, if the Penguins are winning by two goals, then Johnston can balance running all four lines with situational opportunities to make “the most” out of Sill, Adams, and Spaling’s minutes while simultaneously minimizing the opposition's possession opportunities near the Penguins’ own net.

But in cases where the Penguins are down a goal (or the score is tied), and especially after a television timeout (where all four lines have had a chance to rest), Johnston’s de facto decision to skate out the fourth line for offensive zone faceoffs warrants skepticism at best, criticism at worst.

It’s perfectly allowable that Johnston could use his better faceoff men to win draws, establish possession, and then immediately change lines to put the fourth line out in a more-favorable Corsi situation, but to this point, Johnston has not made a habit of quick, adaptive line changes, opting instead to let his players play out their shifts.

Should the Penguins’ offense hit a dry spell, expect to see Johnston flip the faceoff situations, and expect to see more of the Sid-to-Hornqvist snapshot made popular by Malkin and Neal during the past few seasons.


  1. How do shot blocks affect Corsi? Does Adams get a minus for allowing an attempt when he always intended on blocking the shot?

  2. It's based on shot attempts for/against, so something like blocked shots would fall more in line with the Fenwick statistic. No stat (except for my own subjective tracking, which you can find on this website) attempts to factor in "intent" for any given play, so, no, trying to block a shot has no bearing on a Corsi for/against.

    Trust me, I am not a Corsi apologist- Corsi is but A tool for evaluating play, not necessarily THE tool. In this particular case, Corsi explains and justify's (to an extend) Johnston's coaching decisions in the offensive zone, which is why I used it.

  3. A tremendous resource to new "fancy stats" is available via the great folks at War-on-Ice, and they have a great collection of slides that break down these metrics into easy-to-digest bites.

    Also, "justifies." My own spelling errors make me cringe.

  4. Goc has been the center between Sill and Adams more than anyone else by far. I don't understand why you pulled stats as if Spaling has been centering that line.


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