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dCorsi Impact: Is the Oilers' defence headed in the right direction?

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As bloggers in the Corsiverse discuss Peter Chiarelli's moves on defence, we take a look at dCorsi Impact as a metric for measuring the quality of defenders' possession game. Are we headed in the right direction?

Lubomir Visnovsky, dCorsi wonder
Lubomir Visnovsky, dCorsi wonder
Bruce Bennett/Getty Images

The problem with Corsi

It's no secret. There's a problem with Corsi. It's not news either. The analysts have been aware of it's limitations since the very beginning: Corsi doesn't account for context. Perhaps this problem was well, although coarsely, described by former Edmonton Oilers' head coach Dallas Eakins when he somewhat infamously stated:

An individual does not have a Corsi. His Corsi is a whole bunch of players that are with him, right.

Make no mistake about it, Eakins was right, but his statement was also ignorant. Analysts have struggled with this problem in many different ways, and have proposed several tools for dealing with it. Individual players do not have a Corsi but they do have a dCorsi.

Why dCorsi?

A few weeks ago I reintroduced Copper & Blue readers to Stephen Burtch's dCorsi metric and used it to analyze the Oilers' defence. Subsequent to that analysis I took a hard look at the Taylor vs Tyler debate using dCorsi. During the few days that have trickled along since McDavid day our defence has once again become a popular topic of conversation. The purpose of this post is to bring dCorsi into that conversation, and to explore the dCorsi Impact metrics.

The "d" in dCorsi stands for "delta". Although the measure itself is complicated and involves intense calculation, the concept is simple. dCorsi accounts for the contexts that influence Corsi: it provides a measure of independent performance relative to other players in the league.

The dCorsi process first involves the calculation of Expected Corsi scores against which Observed scores are then compared. Expected scores are, for the sake of simplification, average references. Observed scores are regular Corsis, the ones Eakins says individuals don't have, attempted shots that happen on the ice. The delta measure is subsequently calculated as the difference between the Observed and Expected measures .

Although it has yet to achieve universal acclaim within the analytics community dCorsi appears to be a more than adequate tool for comparing individual contributions to possession within teams, across teams, across seasons, and eventually perhaps across leagues and eras. While I'm certain it's not perfect, and I would like too see an extended peer-review of its regression parameters, I believe that Burtch has done something very special and important with this measure. If dCorsi does what it claims to do, and at this point I believe it does, it's useful for grading individual performances regardless of context: it puts the numbers on a level playing field and properly assigns a Corsi number to individuals.

A dCorsi tells us very little about how a player achieved good or bad possession play and nothing about the context that led to it. Context is best investigated through other means such as usage charts an line-mate analysis, the tools that many analysts have developed to account for the known and established issues of Corsi measures. If you want to know about context, don't look to dCorsi. All of that context information is under the hood hidden in Burtch's magnificent engine but the numbers themselves tell us little about context. By comparing dCorsi scores across time and by comparing Expected measures we might be able to glean some of this contextual information but generally speaking what dCorsi tells us about is the player and how the player compares to his colleagues.

dCorsi Impact

Although it doesn't present us with context, dCorsi does separate For and Against measures and such separation is critical. It's various measures tell us that there are several ways of being a possession player. Some players achieve positive scores by moving pucks while others are better at blocking them. The "Impact" measures combine time-on-ice with delta Corsi in order to give us an idea of the influence a player has on the ice.

dCorsi Against Impact (dCAI) is the difference between a player's Observed and Expected Against scores, multiplied by average five-on-five time-on-ice per 60 minutes of play. Negative dCAI is good, and means that a player suppressed attempted Corsis relative to his colleagues. This metric is shown on the horizontal axis for the visualizations in the following section.

dCorsi For Impact (dCFI) is the difference between a player's Observed and Expected Corsi For scores, multiplied by average five-on-five time-on-ice per 60 minutes of play. Positive dCFI is good. dCFI is a relative measure of offence (not to be confused with scoring). This metric is shown on the vertical axis for the visualizations in the following section.

dCorsi Impact (dCI) is the difference between a player's dCAI and dCFI. dCI is an overall relative measure of a players effect of on-ice possession. This metric corresponds to bubble size for the visualizations in the following section.

dCorsi Impact Visualizations

For the following charts and tables all data originates from the war-on-ice Burtch dCorsi lab. All data points combine regular and playoff data for each season, while separating performances from different teams within a season across multiple bubbles. All visualizations are by bituman. While the position of bubbles is consistent between charts, the size of bubbles should only be compared within charts.

Fullscreen version and table here. Mouse-over bubbles to display associated data.

The above visualization is of all Impact data published for NHL defenders. The horizontal axis shows dCAI, while the vertical axis shows dCFI. The dark lines at zero represent average values. Bubble size corresponds to dCI.

There are four quadrants. The top-left quadrant contains the best Impact performances. These have positive For (above the horizontal line at zero) and negative Against (to the left of the vertical live at zero) values. All of the performances in the top-left quadrant are "above average". You may see some outliers in this quadrant. These are mostly the names you would expect to see (Chara, Keith, Hamilton, Boychuck, Giordano, etc).

The lower-left quadrant represents performances that were above-average in shot-suppression, but below average at generating offence. The outliers in this quadrant are players like Olivier Ekaman-Larsson and Mark Fistric.

The top-right quadrant represents defender performances that were above-average at generating offence, but below average in shot-suppression. Dion Phaneuf, Brent Seabrook, and Jason Smith have performances in this quadrant.

The bottom-right quadrant represents below-average defender performances. Included in this quadrant are performances by Johnny Oduya, Robin Regehr, and Justin Schultz.  The charts essentially grade performance by defenders as follows:

A few good men

In order for us to get the feel of what bubble positioning means, I prepared a few charts for individual elite defenders. There are several ways to be an elite defender and it seems that MSM claim that there has been a change towards "puck-moving" defenders is somewhat justified by dCorsi Impact data.
Full screen version and table here. Mouse-over bubbles to display associated data.

Full screen version and table here. Mouse-over bubbles to display associated data.

Full screen version and table here. Mouse-over bubbles to display associated data.

dCorsi and Oilers' defenders

Full screen version and table here. Mouse-over bubbles to display associated data.  Blue represents the 2012-2013 season, orange the 2013-2014 season, and 2014-2105 is represented by red. All other seasons are in grey.

I was surprised to see how Chris Pronger was positioned relative to Chara and some of the other elite defenders. Pronger was a great defensive defender, but compared to Chara, Karlsson, and a few performances by Giordano he did not generate much offence. In this regard Pronger was surpassed by Lubomir Visnovsky on the 2005-2006 Oilers. In my mind I usually associate the 2006 Stanley Cup run with Pronger's contribution, but Visnovsky was incredibly important in that run possession wise as well.

Full screen version and table here. Mouse-over bubbles to display associated data. Blue represents the 2012-2013 season, orange the 2013-2014 season, and 2014-2105 is represented by red. Notice the dearly departed Jeff Petry and Martin Marincin were the Oilers' best possession defenders over the past two seasons. Belov and Fistric also did well considering their deployment.

Full screen version and table here. Mouse-over bubbles to display associated data.  Blue represents the 2012-2013 season, orange the 2013-2014 season, and 2014-2105 is represented by red. All other seasons are white. Please note that seasons included in these visualizations are from multiple teams. From a possession standpoint the Oilers defence has taken a step backwards by losing Petry and Marincin.

Andrej Sekera, although a good possession defender in some seasons, has not put-up dCorsi close to that of Petry or Marincin in the past three years, and has lately been struggling to suppress attempted shots (although his offensive puck moving is still good). When Eric Gryba is deployed properly he's capable of producing average results.

UFA Defenders and Trade Potentials

Full screen version and table here. Mouse-over bubbles to display associated data. Blue represent the 2012-2013 season, orange the 2013-2014 season, and 2014-2105 is represented by red. Other than UFA's I visualized data from defenders mentioned in the comments section of Alan Hull's latest contribution to Copper & Blue. Johnny Oduya doesn't measure very well. Schlemko seems like a good bet.

Bullish on "strong on the stick", bearish on "hockey"?

By letting Petry slide, trading Marincin for little, and picking-up Gryba the Oilers have a weaker defence from the possession perspective according to dCorsi Impact.

Being strong on the stick might be important. But should it come at the expense of having positive possession skills? If names like Chara and Giordano mean anything the answer to that question is "no". Clearly both physical play and proper defensive play are important, at least until the NHL cracks-down on cross-checking and holding in the crease. Several of the league's best defenders are tough SOB's but they also know how to play hockey. When I see Chara in action I'm amazed at the pounding he gives the would-be Ryan Smyths bothering his keeper. I wouldn't last five seconds taking the poundings he routinely hands-out, pounding which most certainly will eventually be found to cause concussions. The criticism that the Oilers need a tougher defence is defensible; they haven't exactly "crushed-it in the crease".

What's baffling, however, are not the moves towards bigger and stronger defenders. It's that these moves come at the cost of Martin Marincin and not at the cost of Andrew Ference who is simply under-average. While Justin Schultz is also firmly planted in the red quadrant his scoring ability redeems him to a certain extent. Ference on the other hand is taking up space that might otherwise be filled by a possession, bone-crushing, or scoring defender. Regarding Nikitin, while I believe firmly that the Oilers should have bought him out, and should either trade him or waive him, by these measures he was deployed properly last season and did OK, not $4-million dollars OK, but that's another story.