Metrics offer insight into Norris Trophy candidacies

Offense tends to carry the day in voting for the Norris Trophy. For the past six seasons, the winner has never ranked lower than No. 2 among NHL defensemen in scoring.
But is offensive proficiency enough this season to win an honor that has been awarded annually to the "defense player who demonstrates throughout the season the greatest all-round ability in the position"?

Voting for the Norris Trophy is conducted by the members of the Professional Hockey Writers' Association, most of whom do not get the opportunity to watch every minute of every game. Modern statistical analysis can be an objective way to confirm observations from those limited viewings, and to help fill in the gaps of what might have been missed. It could be crucial this season in order to separate a crowded field of defensemen.
At first glance, it appears Erik Karlsson of the Ottawa Senators is the best offensive defenseman in the League by a safe margin, but there are several others who score at a similar rate, once opportunity is taken into account.
In absolute terms, Karlsson leads defensemen with 76 points in 76 games, followed by Brent Burns of the San Jose Sharks with 71 points in 77 games, and Kris Letang of the Pittsburgh Penguins, with 59 points in 66 games.

But if they were assigned the exact same ice time as Karlsson at even strength and on the power play, Burns would be projected to lead the scoring race with 80 points, and Letang would match Karlsson's 76 points.
Beyond those three, the scoring race would be tight. With Karlsson's ice time, John Klingberg of the Dallas Stars would have 75 points, followed by Shayne Gostisbehere of the Philadelphia Flyers with 74 and Roman Josi of the Nashville Predators with 72. There is no clear-cut winner.
Another statistical wrinkle is not all of a player's offensive contributions will necessarily be recorded on a score sheet, especially for players on weaker teams.
For example, consider P.K. Subban of the Montreal Canadiens, who won the Norris Trophy in 2012-13. At even strength this season, the Canadiens have averaged 3.06 goals per 60 minutes when Subban was on the ice, exactly double their rate of 1.53 when he was not. That is the largest margin among NHL defensemen this season. Clearly, Subban, out with a neck injury, has found ways to fuel the Canadiens' scoring in ways that didn't always get his name recorded on the score sheet.
But despite the emphasis on offensive production, the Norris Trophy is for all-around ability, so defense has to be a factor.

It's a bit more difficult to statistically determine which defensemen are doing the job on defense, because there's no equivalent statistic for how many goals a player prevented, either directly or indirectly.
Traditionally, PHWA voters consider which defensemen are being used in key defensive situations: killing penalties, taking on high-scoring opponents, being deployed for defensive-zone faceoffs, and defending late leads. That information is weighed against how effectively each defenseman denies his opponents' shot attempts, scoring chances, and goals in those situations.
Ways to analytically measure each of these have all been developed in recent years. Zdeno Chara of the Boston Bruins, who won the Norris Trophy in 2008-09 despite ranking No. 12 in defenseman scoring, is a good example of defensive prowess being taken into account.
The ideal situation for voters is when a player finds a way to hold a top-10 spot among the metrics that define excellence on the offensive side of the puck, and a top-10 ranking in defensive metrics.
Duncan Keith of the Chicago Blackhawks in 2013-14 and Nicklas Lidstrom of the Detroit Red Wings in 2010-11 were two such players to win the Norris.
This season, there is no such player.
Of the six defensemen with 70 or more Karlsson-adjusted points, Josi is the only one who is his team's top defensive option. Whether killing a penalty, facing a top opponent, or protecting a late lead, Josi and defense partner Shea Weber are Nashville's top options in key defensive situations.

The biggest issue with Josi's bid for the Norris Trophy is the Predators aren't particularly effective when he's out there in that role. Specifically, opponents average 57.9 shot attempts per 60 minutes when Josi is on the ice at even-strength, and 47.6 when he is not. Though some increase is expected given the difficulty of Josi's assignment, that extra 10.3 shot attempts allowed per 60 minutes represents the second largest increase among NHL defensemen, behind Dan Girardi of the New York Rangers, with 13.3.
In contrast, this is another area where Karlsson shines. Ottawa has allowed 55.5 shot attempts per 60 minutes when Karlsson has been on the ice, and 61.8 when he is not, one of the best margins in the League. Factor in his additional offense and Ottawa's share of all shot attempts rises from 44.3 percent to 51.3 percent whenever Karlsson is on the ice. That margin of 7.0 percent ranks No. 5 among NHL defensemen.
From this perspective, Karlsson's greatest challenger for the Norris Trophy could come from a top-pairing player from another team. Victor Hedman of the Tampa Bay Lightning, Drew Doughty of the Los Angeles Kings and Oliver Ekman-Larsson of the Arizona Coyotes all feature shot-attempt percentages as strong as Karlsson's, and each could potentially finish at least No. 12 in the defenseman scoring race.