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December 14, 2011
Stats and Fury
Don't Blame The Bench Bosses

by Kent Wilson and ESPN Insider

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Only 30 or so games into the season and already National Hockey League coaches are dropping like flies. The recent firing of Terry Murray brings the 2011-12 total to five (including Davis Payne, Paul Maurice, Bruce Boudreau and Randy Carlyle) and we haven't even hit the midseason mark.

It's often said that coaches are hired to be fired. It's a funny quip that makes light of how frequently the axe falls in the NHL, but looking at some statistical trends in the wake of past coaching changes, it appears that a coach's impact between his hiring and firing is fairly minimal. In truth, the coach is seldom at fault for a team's struggles, and the improvement teams experience after he's canned is not completely because of the new face behind the bench.

Aside from the fact a general is limited by the quality of his troops, so to speak, what we most often see play out in today's NHL is coaches being sacrificed before the percentages can correct themselves into a true representation of the team's talent. More specifically, when a team goes on a bad run because of a below average shooting percentage or save percentage, the coach gets the pink slip.

While it may seem intuitive that coaching would exert some influence on a team's percentages, in fact, team-wide shooting percentage and save percentage tend to vary around league means almost at random in the post-lockout NHL, despite consistent presences behind and on the benches. For instance, here are the top-five and bottom-five teams in terms of even strength shooting percentage (ES SH%) from 2009-10 and the same groupings from the following season:

Top to Bottom
A look at team 5-on-5 shooting percentages from one season to another.

Top 5 2009-10 		Top 5 2010-11 	Bottom 5 2009-10 	Bottom 5 2010-11
Capitals: 11.0% 	Flyers: 9.2% 	Bruins: 6.9% 		Devils: 6.7%
Canucks: 9.8% 		Stars: 9.1% 	Red Wings: 7.2% 	Canadiens: 7.0%
Avalanche: 9.4% 	Blues: 8.8% 	Maple Leafs: 7.6% 	Senators: 7.1%
Thrashers: 9.2% 	Coyotes: 8.7% 	Flyers: 7.7% 		Panthers: 7.3%
Sharks: 9.1% 		Bruins: 8.7% 	Flames: 7.9% 		Thrashers: 7.4%

This illustration is somewhat anecdotal but still instructive -- none of the best or worst teams in terms of scoring frequency repeated their respective feat the next season. In fact, teams such as Atlanta and Boston found themselves at the other end of the spectrum -- the Thrashers had the fourth-highest 5-on-5 shooting percentage in Season 1 above, but the fifth-lowest in 2010-11. The Bruins, in contrast, went from the worst shooting team in the league the first season to the fifth-best the next.

The Washington Capitals scored one of the highest goal totals (313) in recent memory in 2009-10 on the back of their remarkable 11.0 percent even strength shooting percentages. But even a team featuring the high-octane offense of Nicklas Backstrom, Alex Ovechkin, Alexander Semin and Mike Green couldn't sustain anything close to that eye-popping number. The next season, with the same coach and mostly the same roster, the Capitals dropped to a below-average 8.1 percent (eighth-lowest that season).

Over the long run, clubs will tend to regress to the mean. Hockey Prospectus author Jonathan Willis looked at the season-over-season correlation of teams' even strength shooting percentage stretching from 2007-08 to 2010-11. He found there is essentially no relationship. A club's shooting percentage one season tells you nothing about what it will be a season later. The same is true to some degree for save percentage, though a routinely elite goalie and a routinely awful goalie will mitigate the randomness somewhat.

The takeaway from all this is that the percentages vary largely independent of coaches and personnel. Some teams are just scoring (or saving goals) at a worse percentage than they would be expected, given their talent levels.

The difference between a few percentage points in goals or saves doesn't seem very large, but we're talking about thousands of shots over the course of a season. The difference between an 8.0 and a 9.0 shooting percentage over, say, 2,000 shots is 20 goals (180 versus 160). That can sometimes be the difference between a coach keeping his job and getting fired.

Of course, the percentages tend to bounce around even more erratically in the short term. Teams can run exceptionally hot or cold in relatively small samples, but inevitably the percentages regress toward the mean. But often front offices and owners don't believe they have the time to wait it out. It is often these cold snaps that cost an NHL bench boss his job. And conversely it helps build the reputation of his successor when the correction begins.

For example, Payne, the first coach axed this season, was done in by poor goaltending and a bad shooting percentage on the power play. Although the Blues have a much better record under Ken Hitchcock, the truth is they are mostly the same club they were before:

Blues under Payne 		Blues under Hitchcock
Even strength SH%: 7.9 		Even strength SH%: 7.0
Even strength SV%: 92.6 	Even strength SV%: 96.3
Even strength shot ratio: 55.6% Even strength shot ratio: 54.2%

The difference is Jaroslav Halak and Brian Elliott posting above-average save percentages in eight games each for Hitchcock (95.2 percent and 97.4 percent, respectively) while Halak struggled mightily for Payne at the start of the season (89.5 percent).

Shot ratios (for/against) tend to be far more stable and therefore a better indicator of a team's true ability. Strong teams tend to dominate shot rates and puck possession in the NHL, while weaker teams tend to spend more time in their end and get outshot. If there is a true "signal" to be found in the noise of results to differentiate one coach from another, it can be found in shots for and against.

We can repeat the Payne exercise to see what effect, if any, the other coaching changes have had this season. Carlyle's Ducks were poor at everything -- in volume of goals stopped and scored as well as shooting ratio at even strength. The club has been better as a matter of chance under Boudreau (improved percentages) for five games, but it is still getting grossly outshot at 5-on-5. No true positive effect there so far, though its ES SH% is up to 11.1 percent from a meager 7.1, and the save percentage has risen 1.1 percent.

In Washington, the Caps' shooting percentage has actually dipped 0.3 percent since Boudreau's departure, but a 1.4 percent improvement between the pipes has helped current coach Dale Hunter's crew capture a few wins. But there's a warning sign in Washington: The Caps' shot ratio has plummeted in their six games under Hunter. It's way too early to denounce the new coach just yet, but the early returns suggest the team was better under Boudreau.

So where does this leave the Kings?

Under Murray, L.A. posted a shooting percentage of 5.8 percent and a save percentage of 93.1 percent. They even had a shots ratio that tipped in their favor at 51.3 percent. The culprit here is clear: an unsustainably bad even strength shooting percentage.

It's arguable the Kings should be outshooting their foes more frequently, given their strong roster, but the primary reason Murray was let go is that ugly 5.8 number. Expect the Kings' goal rate to go up as a matter of regression going forward, no matter who is named the next permanent Kings coach.

A version of this story originally appeared on ESPN Insider Insider.

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