Reasons Behind the Failure to Accurately Evaluate NHL Defensemen

Constructing a roster full of talent is necessary for success in the NHL as it undoubtedly is in all team-based professional sports. Evidently, it is also important that a coach deploys his artillery of talent in an optimal fashion. The magnitude of a player’s impact is largely a product of opportunity. In hockey, opportunity comes in a few different forms – the most prominent one being ice time.

Now consider what we know about useful possession. Teams with better shot-attempt differentials are more likely to win a larger percentage of future games than those with poor shot-attempt differentials. Why? Because they are still capable of winning when the percentages don’t swing in their favor. It is a coach’s job to deploy his roster in a way that optimizes his team’s Corsi differential and consequently improves their chances of success. In order to do so, better possession players need to be the beneficiaries of additional even strength ice time.

Below are a couple of charts that show the relationship between even strength ice time per game and Usage Adjusted Corsi% for forwards and defensemen sine 2007-08 (500+ 5v5 TOI)…

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It seems as though NHL coaches have historically done a really poor job allocating ice time to defensemen who tilt the ice in the right direction. This is staggering considering that defensemen have virtually no bearing over on-ice percentages and therefore their possession rates are particularly illustrative of their effectiveness. The issue likely stems from a few misconceptions surrounding which attributes make a defenseman effective. There are obviously kinks in the evaluation process and the point of this piece to bring to light a few of these fatal fallacies.

DEFENSE IS NOT LIMITED TO THE DEFENSIVE ZONE

In hockey, defense is used to describe anything that contributes to a team’s ability to reduce the rate at which goals are scored against. The best way to reduce picking pucks out your own net is to suppress the opposition’s rate of chances and attempts for. Jen Lute Costella (@RegressedPDO on Twitter) gave an outstanding presentation about shot suppression at the Pittsburgh Analytics Workshop back in November. Her point was basically that anything that increases the amount of time between your opponent’s opportunities technically qualifies as shot suppression. That includes…

Defensive Zone

– Zone coverage

– Zone exits

Neutral Zone

– Preventing zone entries

– Entering the opposing team’s defensive zone with or without control

Offensive Zone

– Sustaining offensive zone pressure

– Fore-checking (less applicable to defensemen)

As you can see, defense happens all over the ice.  Too often we see NHL coaches lean on big physical blue liners like Brooks Orpik, Andrew MacDonald, Rob Scuderi etc who lack the ability to move the puck and sustain offensive zone pressure – the archetypal shut-down defenseman. Regardless of whether or not these types of defensemen are effective in their own zone, they often spend far too much time defending and as a result, rack up attempts/chances against at a very high rate.

PHYSICALITY/GRIT IS NOT A PROXY FOR DEFENSIVE EFFICACY

This one ties also into the idea that defense is limited to the defensive zone. Coaches often attribute physicality to being reliable defensively but as noted earlier, defensive play goes far behind pinning an opposing forward along the wall and clearing the crease. To prove it we can look at hit totals for defensemen and test the correlation with goals against per 60.

Since defensemen who spend more time in their own zone tend to hit more frequently, we will adjust hits for possession by dividing hits by Corsi against

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A correlation coefficient of 0.20664 suggests that there is a link, albeit a weak one, between hits per Corsi against and goals against per 60. This means that physicality does play a role in reducing a defenseman’s goals against (which really isn’t surprising). The issue in hockey is that the relationship between physicality and defensive ability is perceived as being much stronger and that leads to sub-par defensemen being over-utilized. The great thing about Corsi is that it is a macro-statistic and therefore a gritty defender’s ability to reduce attempts against using his size is already captured within shot attempt measures. This dilemma is very similar to the over-emphasis of a Center’s ability to win face-offs: causal factor with a marginal influence on Corsi differential.

PRODUCTION IS MORESO A PRODUCT OF OPPORTUNITY THAN IT IS SKILL

The objective in hockey is to out-score your opposition and thus it makes sense that production is such a highly sought after commodity in the NHL. The hockey analytics movement has changed the way we measure point production by shifting from units per game to units per 60 minutes. This way we can even-handedly compare players who receive differing amounts of ice time per game. Just like most measures in hockey, point scoring is also subject to random noise from season to season as on-ice shooting percentages and individual point percentages flux uncontrollably.

The following charts tests the repeatability of 5v5 Points/60 from season-to-season since 2007-08 (500+ 5v5 TOI)…

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As you can see, even strength production is much more repeatable for forwards than it is defensemen. That shouldn’t be surprising given the fact that high scoring forwards do have the ability to sustain high on-ice shooting percentages to a certain degree whereas high scoring defensemen do not. Here is another look at how 5v5 Points/60 regresses to the mean from season to season for forward and defensemen (each bucket contains 150 skaters)…

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Regardless of his rate of production in year 1, a defensemen should be expected to finish within 0.60-0.85 points per / 60 at even strength in his subsequent season. That isn’t a large spread. The disparity we see between defensemen who consistently finish with a large sum of points year over year compared to those who don’t is largely the result of opportunity in the form of ice time.

CONCLUSION

The reality is that the National Hockey League is just starting to scrape the surface of analytics and their application to the decision-making process. A variety of inefficiencies still persist waiting for teams to either exploit or fall victim to and that definitely includes the way the market assesses defensemen. As time progresses along with conceptual understandings, the line between the terms offense and defense will become blurred and thus the evaluation of blue liners will drastically imrpove. For now we wait and pull our hair out as the Dion Phaneufs and Marc Staals of the world sign off on massive contracts and continue to be excessively deployed and relied on to “shutdown” opposing teams’ top lines.

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