Thursday, December 11, 2014

The best NHL players create higher quality shots

A couple of months ago, I pointed to data that showed team shot quality is a real characteristic of a team, and not just random noise which the hockey analytics consensus believes it to be. 

That had to do with opposition shot distance. In 2013-14, the Wild allowed only 32 percent of opposing shots from in close, as compared to the Islanders, who allowed 61 percent. Those differences are far too large to be explained by luck.

Here's one more argument, that -- it seems to me -- is almost undeniable evidence that SH% must be a real team skill.

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It's conventional wisdom that some players shoot better than others, right? In a 1991 skills competition, Ray Bourque shot out four targets in four shots. The Hull brothers (and son) were well-known for their ability to shoot. Right now, Alexander Ovechkin is considered the best pure scorer in the league.*

(*Update: yeah, that's not quite right, as a reader points out on Twitter.)

In 1990-91, Brett Hull scored 86 goals with a 22.1 percent SH%. Nobody would argue that was just luck, right? You probably do have to regress that to the mean -- his career average was only 15.7 percent -- but you have to recognize that Brett Hull was a much better shooter than average.

Well, if that's true for players, it's true for teams, right? Teams are just collections of players. The only way out is to take the position that Hull just cherry-picked his team's easiest shots, and he was really just stealing shooting percentage from his teammates. 

That's logically possible. In fact, I think it's actually true in another context, NBA players' rebounds. It doesn't seem likely for hockey, but, still, I figured, I have to check.

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I went to hockey-reference.com and found the top 10 players in "goals created" in 2008-09. (I limited the list to one player per team.)  

For each of those players, I checked his team's shooting percentage with and without him on the ice, in even-strength situations in road games, the following season. (Thanks to "Super Shot Search," as usual, for the data.)  

As expected, their teams generally shot better with them than without them:

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With  Without
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10.1   5.8   Ovechkin
 5.5   9.3   Malkin
 7.1   5.3   Parise
 6.0   8.2   Carter 
10.3   9.1   Kovalchuk
 8.9   5.7   Datsyuk
10.8   7.5   Iginla
10.7   7.2   Nash
 9.8   7.5   Staal
 9.0   9.1   Getzlaf
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 9.4   7.5   Average
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Eight of the ten players improved their team's SH%. Weighting all players equally, the average increase came out to +1.9 percentage points, which is substantial. 

It would be hard to argue, I think, that this could be anything other than player influence. 

It looks way too big to be random. It can't be score effects, because these guys probably play roughly the same number of minutes per game regardless of the score. It can't be bias on the part of home-team shot counters, because these are road games only. 

And, it can't be players stealing from teammates, because the numbers are for all teammates on the ice at the same time. You can't steal quality shots from players on the bench.

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I should mention that there was also an effect for defense, but it was so small you might as well call it zero. The opposition had a shooting percentage of 7.7 percent with one of those ten players on the ice, and 7.8 percent without. 

That kind of makes sense -- the players in that list are known for their scoring, not their defense. I wonder if we'd find a real effect if chose the players on some defensive scale instead? Maybe Selke Trophy voting?

Also ... what's with Malkin? On offense, the Penguins shot 3.8 percentage points worse on his shifts. On defense, the Penguins opponents shot 3.8 percent better. Part of the issue is that his "off" shifts are Sidney Crosby's "on" shifts. But even his raw numbers are unusually low/high.

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Speaking of Crosby ... if you don't believe that the good players have consistently high shot quality, Crosby's record should help convince you. Every year of his career, the Penguins had higher quality shots with Crosby on the ice than without:

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With  Without 
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11.7   9.4   2008-09 Crosby
10.1   7.1   2009-10 
 9.6   6.9   2010-11
13.9   7.3   2011-12
13.2   8.8   2012-13
10.4   6.5   2013-14
 7.1   7.0   2014-15 (to 12/9)
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10.9   7.6   Average
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Sidney Crosby shifts show a consistent increase of 3.3 percentage points -- even including the first third of the current season at full weight. 

You could argue that's just random, but it's a tough sell.

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Now, for team SH%, you could still make an argument that goes something like this:


"Of course, superstars like Sidney Crosby create better quality shots. Everyone always acknowledged that, and this blog post is attacking a straw man. The real point is ... there aren't that many Sidney Crosbys, and, averaged over a team's full roster, their effects are diluted to the point where team differences are too small to matter."

But are they really too small to matter?  How much do the Crosby numbers affect the Penguins' totals?

Suppose we regress Crosby's +3.3 to the mean a bit, and say that the effect is really more like 2.0 points. In 2013-14, about 38 percent of the Penguins' 865 (road, even-strength) shots came with Crosby on the ice. That means that the Crosby shifts raised the team's overall road SH% by about 0.76 percentage points. 

That's not dilute at all. Looking at the overall team 5-on-5 road shooting percentages, 0.76 points would move an average team up or down about 8 positions in the rankings.

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Based on all this, I think it would be very, very difficult to continue arguing that team shooting percentage is just random.

Admittedly, that still doesn't mean it's important. Because, even if it's not just random, how is it that all these hockey sabermetric studies have found them so ineffective in projecting future performance? 

The simple answer, I think, is: weak signal, lots of noise. 

I have some ideas about the details, and will try to get them straight in my mind for next a future post. Those details, I think, might help explain the Leafs -- which is one of the issues that got this debate started in the first place.



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1 Comments:

At Friday, December 12, 2014 9:37:00 AM, Anonymous Anonymous said...

What percent of hockey goals come on the initial shot on goal, and what percent come on the 1st, 2nd, 3rd...rebound of the initial shot?

 

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