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Bruins' ups and downs a function of shooting percentage

Posted by Andrew Mooney  March 27, 2012 01:00 AM

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Following a 9-0 demolition of the Calgary Flames on Jan. 5, the Bruins stood in sole possession of second place in the Eastern Conference, one point behind the first-place New York Rangers. They looked like contenders to repeat as Stanley Cup champs. I didn’t think they could be — but then the bottom fell out. Since that game in early January, the Bruins have gone 18-18-2, dropping them to fourth in the conference with 91 points. What caused this sudden dropoff? An investigation of the Bruins’ erratic offensive output may provide the answer.

During their floundering start to the year—which I’ll define as the 3-7-0 month of October—the Bruins’ raw shot totals were not markedly different from their numbers the rest of the season; they averaged 33.4 shots a game over this period, as compared to 33.0 for the entire year.

Then came the torrid months of November and December, in which the Bruins raced back into first place in the Northeast Division behind a 21-3-1 record. They dominated on both ends of the ice, averaging 3.96 goals per game (scoring 5 or more goals 11 times) while allowing a mere 1.68 goals against. The rest of the NHL was on notice: play Boston, and prepare to get steamrolled.

We’ve seen what’s unfolded the last two months, during which the sputtering Bruins have a sub-.500 record. As of their March 19 victory over Toronto, their scoring average had dropped to 2.66 goals per game, and their opponents had outscored them by almost a half-goal per game (3.09 goals against)—a far cry from their previously dominant play.

However, the Bruins’ earlier success may have been distorted by their high even-strength shooting percentage, with which their fluctuating fortunes have trended closely. Their shot totals during November and December had not noticeably changed from their early-season slump; in fact, they had even dropped slightly, to 32.2 per game. Yet the Bruins’-even strength shooting percentage had risen all the way to 12.1 percent, up from 5.8 percent in October. For a bit of perspective, the team with the highest even-strength shooting percentage in 2010-11 was the Philadelphia Flyers, at 9.2 percent, and the league average typically hovers around 8.0 percent.

During the current decline, that percentage has again dropped fairly steeply, down to 6.8 percent. The Bruins are actually getting more shots on net (33.5 per game) than in either of the previous two stretches, but they’ve found the back of the net far less often.

Interestingly, the Bruins’ shooting percentages have varied inversely with those of their opponents in each of these three time periods—in other words, as the Bruins struggled to find the net, their opponents did so comparatively easily, and vice versa. In October, Boston opponents shot a respectable 8.4 percent; in November and December, they shot just 5.1 percent; and in the present slide, 10.8 percent.

So what does this mean? One might think the Bruins were simply playing worse hockey, and maybe they were. During their rough patches, they may not have been getting the same quality shots with which they routinely racked up 5- and 6-goal games during their winning streak.

But that’s not the whole story; the change could just be the result of luck. The work of various hockey researchers has suggested that it’s much easier for teams to control the shot total of their opponents than the quality of those shots — that is, in aggregate, most shots on goal are indistinguishable in value. Since the shot totals of the Bruins and their opponents have been largely stable throughout the season, the hot and cold goal-scoring stretches can be attributed to random variation. The Bruins were due for a regression to the mean after November and December, and sure enough, they cooled off to start 2012.

I was first tipped off to this trend by an article from Grantland’s Katie Baker, who referenced the work of NHL blogger Cam Charron. Charron’s piece sums things up nicely.

Since … 2008, when a small Canadian website began recording NHL.com play-by-play data, only two teams, the 2009 Pittsburgh Penguins and the 2010 Washington Capitals, have finished with shooting percentages of higher than 10 percent. Boston’s 9.8 percent shooting rate by the time they hit the White House break wasn’t just due to come down: it already was dropping.

With four wins in their last five games, the Bruins appear to have righted the ship, at least for now. Their even-strength shooting percentage for the season has returned to a reasonable 8.7 percent. And as the cream of the Eastern Conference is concentrated in the Atlantic Division (New York, Pittsburgh, and Philadelphia), the Bruins are currently in line for a No. 2 seed in the playoffs. Still, given the improvement in the conference this year, they may need more of that shooting magic if they’re to make another deep run this playoff season.

As the old saying goes, you’d rather be lucky than good.

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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Stats Driven is powered by David Sabino, who over the last two decades has been a source of statistical analysis on the pages of Sports Illustrated, New York Times, and Chicago Tribune. David has written about all seven recent Boston-area championships for Sports Illustrated Presents commemorative issues, was the creator of such long time features as SI’s Player Value Ranking, NBA Player Rating and long running fantasy football and baseball columns.

He has also authored or made contributions to many books, including the Sports Illustrated’s 100 Fenway: A Fascinating First Century.

Now living in Marblehead, he’s focusing his attention on the Boston sports scene, specifically delving into the numbers affecting the Red Sox, Patriots, Celtics and Bruins, with the goal of informing and entertaining real fans. You can follow him on Twitter at @SabinoSports.

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