Friday, November 06, 2009

The Problem With "Shots on Net"

Shots on net is a terrific statistic, especially at the NHL level. We know that as early as the 1980s that the big three of Roger Neilson's "second generation stats" for individual players were:
  • ice time
  • scoring chances
  • shots on net[Team]
All broken down by situation, of course (5v4PP, 5v5, EN, etc).

So, after a game against the Oilers in 1989, Lindy Ruff would get a postgame scorecard showing his ice time in different situations, how the scoring chances fell when he was out there, the scoring chances for which he was more responsible, etc. He'd also get a grade for shots-on-net[NYR] and shots-on-net[EDM], again by situation. There would be a whack of other information on there as well. You don' have to be particularly clever to start noticing patterns.

The question is, why use shots on net instead of shots AT net. Or put another way, Corsi+ & - or Shots + & -?

Intuitively you would think that Corsi would be the better indicator of territorial advantage in a single game, or any small sample, like a playoff series. To illustrate that; in the first game of the season the Oilers widely outshot the Flames, but only narrowly outchanced them and narrowly outCorsied them. The Flames missed the net a lot with their chances. Simple as that. Still, some players are far better at getting their shots through on net than other guys, and surely that has value, and that player is more likely to aid in territorial advantage than a guy whose shots tend to whistle wide or get blocked a lot more.

On a team level, they are bunched closely together in terms of "ability to get shots through". Shots+ divided by corsi+. But clearly there are different abilities at the team level as well. The strange thing is that teams who tend to get shots through also had a strong tendency to see the opponents do the same against them. That's completely counterintuitive, so I checked just using data from when the score in the game was tied ... still there.

The relationship, by way of Pearson Correlation, r=.43. Coincidence is unlikely, and though it seems that 90% of callers to sports talk radio are amateur psychologists, I personally find it hard to believe that the ability of one team to get their shots through is having an impact on the ability of the opposition to do the same at the other end of the ice. So before we start digging into the VMM (Vulcan Mind Meld) effect of two teams, we should look for a boring, rational explanation.

The terrific JLikens showed us that there is very measurable home recording bias in 'shots on goal', at least when you look at a large enough time frame. This even though surely some of the scorers have changed over the years.

That's understandable. I'm sure I could find a video clip of a shot at net where the shooter tried to fire it through the defenders legs, the puck glanced off of the Dman's shin pad and then the goalie snagged it with a high glove save. Was it a save or was it going wide? If you think it was wide is it a block or a miss? We could run a poll and get a spread of results from readers here, and we could find another similar clip and do the same. It's likely that the same folks would repeat their opinions.

The one thing we would all agree on, however, is that the puck was shot AT net. So let's look at just the road games.

ON THE ROAD:

On the road the Oilers got 43% of their shots through onto the net at EV when the score was tied. Their opponents got 45% through. The league average is 45%.

Now do the same for the other 29 teams and plot it out. Completely random. If you can see a trend you probably should get a CAT scan. r=.03, which is closer to zero than you would expect from two random sets of 30 numbers.

AT HOME:

On Rexall ice the Oilers got 2.5% more of their shots through, so did their opponents.

The scatter plot for all 30 teams on home ice is below. Click to enlarge, you should be able to read the team names with some effort.


The pattern is obvious, there is a Pearson correlation of r=.70.

The wildly generous scorer is from Chicago. This creates the illusion that Chicago was/is a shot-happy team. In truth, in terms of total shots AT net in the games (for both teams combined), they were nearly spot on the same as the Oilers. The difference is that the Hawks play a lot more in the offensive end of the rink than the Oilers do.

I hope that the coaching staff and management of the team you cheer for realizes this. The management of the team I cheer for recently signed Khabibulin to a big ticket deal and more recently educated the local media on how Quenneville's Blackhawks rack up gaudy shot totals because "they just shoot from anywhere".

7 Comments:

Blogger Scott Reynolds said...

Thanks Vic. A real tear lately. I was curious about the team at the very bottom of the graph. I'd guess that it's Minnesota off the top of my head but I'm really not sure. Who is it?

As to your point, my goodness. How many more reasons are there that the Khabibulin signing was awful.

11/07/2009 3:19 pm  
Blogger Vic Ferrari said...

Scott. I changed the chart to include the team names. After reading your comment (long after) I realized that I had shown "% of shots that didn't get through" instead of the the opposite. So the chart is flipped now as well.

11/08/2009 7:08 am  
Blogger Scott Reynolds said...

Thanks Vic. I sure was wrong about Minny being on the extreme. Having the chart flipped helps the whole thing make more sense too since Chicago is way off by itself as opposed to Toronto which has three neighbours not far off. And I didn't guess any of those four. Dang.

11/08/2009 4:25 pm  
Blogger Hawerchuk said...

Vic,

I noticed some weird things in Chicago, but my hazy memory suggests that they were undergenerous (that's not a word) on missed shots as opposed to overgenerous (that's a word?) on SOG. Thoughts?

11/13/2009 12:55 pm  
Blogger Vic Ferrari said...

How does a scorer not notice a "shot directed at net" though?

I'm sure that some teams play a more uptempo style of game at home, which almost always results in more shots on goal and more shots at net. Especially for teams trying to draw more fans.

Probably not for teams with hardass coaches and GMs (CGY, VAN, MTL, N.J, ANA, etc), probably the opposite for them. Though I've never checked.

But the missed shots just disappear in Chicago. For both teams. Some of the blocked shots too.

PTTS effects can have a big effect on home/road shots at net too. Depending on how well a team did at home (how often they were clinging to a lead in the third or playing with a comfortable lead earlier), and depending on how that team likes to play to the score. That can really skew things from raw totals for home/away shots.

But I find it hard to believe that there would be a visible difference in corsi totals if the scorers had all switched teams last season.

Whether a puck was 'directed at net' ... we're almost always all going to agree. Whether is was a miss, block or save ... plenty of room for disagreement there.

11/14/2009 10:49 am  
Blogger Hawerchuk said...

I'm not sure what goes through the scorer's mind. We don't know why they get shot locations so wrong in New York. For missed shots, perhaps the Chicagoans are only counting shots that miss 'close'? It's strange that there isn't a uniform definition of these things across the league and that they don't do a bit of quality control.

11/14/2009 9:08 pm  
Blogger Canny Cyclist said...

I would tend to agree with Hawerchuk. The Chicago and Toronto numbers for actual shots on goal don't seem way off, it's the misses that seem to be the corrupt data. I have trouble believing that Toronto and their opponents both direct thousands more shots at the net in Toronto than elsewhere, AND that they miss much more frequently.

11/17/2009 11:53 am  

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