Sunday, October 20, 2013

Corsi, shot quality, and the Toronto Maple Leafs, part II

A few follow-ups from the last post about the Leafs and Corsi.  

1.  A few  writers published good responses of some of my arguments.  Here's Eric T, here's Draglikepull, here's Nick Emptage, and here's Cam Charron.  They all pretty much disagreed with me; they think the Leafs' shooting was just luck.  

Oh, and this 2010 post from Tom Awad also notes a negative correlation between shots and shooting percentage.

2.  For every NHL game, the ESPN website provides a diagram of the location of every shot for both teams.  On one internet forum, "Leo Trollmarov" posted those diagrams for the Leafs' first seven games this year.  Here's one, from the Colorado game:

I noticed that the Leafs seem to have had more close-in shots than you'd expect.

Take the two diagonal "trapezoid" lines behind the net, and extend them until they meet in the high slot.  Along with the goal line, they now form a triangle where shots are presumably more dangerous.  At least I hope they're more dangerous, since my argument mostly depends on it.

In those seven games, the Leafs were outshot 242 to 199 overall.  However, in "triangle" shots, it was only 58-57.  (In the game above, I count 11 to 4 for the Leafs, although that's a bit misleading because the Avalanche had a few just outside the triangle.)

In other words, even though the Leafs are being outshot overall, they ran almost even in that category of higher-quality shots.

Now, in fairness, I'm cherry-picked my definition of "higher-quality," when I chose that triangle.  I checked a couple of other things, to see if there were patterns -- like, shots from the side boards, and shots from the point.  Those differences seemed minimal.  But the "triangle" effect was significant.  And, even if those shots aren't higher quality, if the Leafs have a different *pattern* of shots, that's something too.

Um, but there's the problem of small sample size.  

Here, let me check the eighth Leaf game ... oops.  The Leafs were out-triangled 11-4, it looks like.  So, yes, maybe the whole thing is just small sample size.  But an effect like that is what I'd be looking for to support the "Leafs got more quality shots" hypothesis.

3.  In his post, Nick Emptage cited a few games from last year where the Leafs were badly outshot or out-Corsied, but still won.  For instance, Nick said,

"In three games against New Jersey, the Leafs were outshot 75-43 at even strength, and were out-Fenwick-Close’d 92-41. ...They took six points from these games."

For two of those three games, the shot quality looks about even.  But, if you look at the ESPN chart for the first game, the one on March 4, that one looks like the Leafs really did have better shots than the Devils.  Toronto won 4-2, despite being outshot 30-23.  But the Leafs actually had five "triangle" shots to the Devils' three.  Also, if you count longer shots, taken from between the blue line and the back of the faceoff circles ... well, the Leafs took only three of those (what I presume are) low-percentage shots.  New Jersey took *seventeen*.

Looking at some of the other games Nick cites ... against Florida, on March 26, the Leafs were outshot 42-31.  But they were equal in triangle shots, 10-10.  And Florida took 19 long shots to Toronto's 10, which accounts for most of the difference.

And, February 7 against Winnipeg, where the Leafs won 3-2 but were outshot 25-18 ... that one is the most obvious.  The Leafs had eight triangle shots to Winnipeg's three.  And, Toronto had only one long shot to Winnipeg's 11.  

Overall, I looked at seven of the games Nick cited.  Four of them did look like shot quality wasn't a factor, and the Leaf win was probably luck.  But, those other three games, I think you could argue that the Leafs won because, in part, the Leafs had better, albeit fewer, shots on goal.

Now, I'm not sure if this means anything for my theory.  Because, if you look at any game where one team won despite being outshot, there's a pretty good chance they had better shots, even if shot quality is random.  That's because you're cherry picking the anomaly games.

Also, if you look, you might be able to find games that go the other way.  There actually weren't that many games where the Leafs lost despite having more shots on goal.  Maybe this Carolina game qualifies ... The shots were 42-39, but the score was 1-4.  But ... although the Leafs had more long shots than the Hurricanes, they also had more triangle shots.

I dunno, but, just looking quickly, it doesn't seem that farfetched that the Leafs are taking closer shots than their opponents.  But, you'd have to look at all the games, not just my selective sample.  And, I suspect, there are analysts out there who study shot quality on a regular basis, and I haven't seen anyone say that the Leafs were different than normal.

But, just sayin' ... from this, the shot quality hypothesis at least seems *plausible*, doesn't it?

4.  Last season, the Leafs had a higher shooting percentage in 32 of their 48 games.  That is, if you picked the winner by who had the higher shooting percentage, the Leafs would have been 32-16 (.667) on the season.

Can that be all score and power play effects?  Doesn't seem like it.  Well, maybe it could, with some luck added on.  I'll just throw that on the "hmmm..." pile with everything else.

5.  In my post, I found a negative correlation between Corsi and shooting percentage, which supports my argument that there's a tradeoff between quantity and quality of shots.  However, in his post, Eric T. suggested that might just be a reflection of different score ratios.  Teams take fewer (but better) shots when ahead by two goals, so the correlation might just be the fact that teams vary in how often they're in that situation.

He has a point.  To take one example, the Leafs played around 750 minutes with the score tied last year; Ottawa played 1,071 minutes.  Toronto played 954 minutes with the lead, while the Senators played only 622.  

So, I checked the six-year correlation using the "game tied" Corsis.  And it disappeared: it went from -0.22 down to -0.02, which is effectively zero.  I hadn't noticed that, because I had only checked last season, when the correlation was -0.24.  In fact, it was also -0.24 the year before.  But, in each of the four years prior, there was actually a *positive* correlation:

2007-08: +0.08
2008-09: +0.02
2009-10: +0.02
2010-11: +0.01
2011-12: -0.24
2012-13: -0.24

I'm not sure what to make of this, whether it's just random, or whether something changed the last two years.  But, I have to admit, this makes my hypothesis a little less justifiable.  

So, I thought it would be good to look at more data.  And, just a couple of hours ago, Nick was kind enough to send me team shot/goal data back to 1997-98.  (Thanks, Nick!)

Omitting last season, the correlation between shot quantity and shooting percentage since 1997-98 was -0.13 (not adjusting for season).  That seems legitimate, and reasonable in magnitude.  (It could be due to score and power-play effects, of course.)

But, last year ... that was different.  Last year, the correlation was -0.52.  

That is HUGE shot quality effect.  Now, when you switch to 5-on-5 only, the correlation drops to -0.42.  And, as we saw, when you switch to Corsi and tied situations only, it drops further to -0.24.  But, still.

Why so high?  In part, it could be the shorter season -- maybe score effects and power play effects didn't have enough chance to cancel out, and we're just seeing artificial differences.  But ... -0.52?  That high?  It doesn't seem like that could be all of it.

Is there something unusual about last season?  

(There are seven parts. Part I was previous. This is Part II.  Part III is next.)

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At Sunday, October 20, 2013 7:13:00 PM, Anonymous Nick Emptage said...

A few thoughts:

1. First off, thanks very much for linking to and responding to my work. Much appreciated. And no worries about the shot data.

2. Regarding the huge shot-quality effect from last season, maybe a dumb question: have you checked to see how sensitive these correlations are to outlier teams? That is, how much does the relationship change if you remove 1 or 2 extreme teams from a season's analysis? (i.e., how strong is the correlation in 2013 without the Leafs' data?)

3. Regarding your thoughts on shot quality, here's my thinking: teams with more top-end skill at forward will average a decent number of "triangle shots" per game, but bad possession teams won't get many "non-triangle" (i.e., low quality) shots, because they spend a lot of the game (a) defending their own end and (b) dumping the puck in to change. It doesn't surprise me that the Leafs can get a decent number of triangle shots given their skill on the wings.

4. I made this point in my post, but the scorching pace of the Leafs game in 2013 makes it hard for me to believe the notion that they were playing a deliberate "defend and counter" strategy that emphasized high-quality shot creation at the expense of shot differential. It's not that high-skill teams can't play excellent defense; the 2007-08 Red Wings are probably the best example of this. But the Wings'
approach was actually a low-event one that focused on shot prevention; that team actually played a pretty slow game. That the Leafs were a terrible shot-prevention team regardless of the score tells me that they weren't playing that style by choice.

At Tuesday, October 22, 2013 12:37:00 PM, Blogger Phil Birnbaum said...

Nick, haven't forgotten you. will reply to this soon.

At Thursday, October 24, 2013 7:59:00 AM, Anonymous Joe W said...

Last season had no interconference play. Over the last few seasons the West has been stronger than the East in winning games and in Corsi - except for last year when both conferences were equal because they never played each other in the regular season. I don't know if this would affect the correlations but it might be worth examining the conferences separately.

Also, isn't last season's change in the correlation between shots and SH% what we would expect to see if teams started making coaching and/or front office decisions based on shots rather than goals? If some teams start focusing on shot quantity at the possible expense of shot quantity? Could be something to watch this year.

At Tuesday, October 29, 2013 12:40:00 PM, Anonymous Steve Burtch said...

Traditionally rather than "Triangle" shots the area used is the "Home Plate" which is the region from the top of each faceoff circle down to the base of each faceoff circle in to the corners of the net (which gives you a shape that looks like home-plate - or "the house" where people take the puck to on a net drive).

Using this definition Greg over at Some Kind of Ninja has created a scoring chance counter that displays goals and shots.

If you look at the Leafs counts for and against last year in total they have 314 scoring chance shots For in the home plate area and they shot 17.4% on them. They allowed 395 shots AGAINST from the home plate area and their opposition shot 15.1% on them.

They're giving up more scoring chances than they're getting in all likelihood - even if we use a method of calculating based on WHERE the shots are taken from.

None of this supports your theory... but if we keep bouncing around this idea we'll get there eventually? maybe? (doubt it).

At Tuesday, October 29, 2013 12:43:00 PM, Anonymous steve burtch said...

To your question about something unusual last season - I'd imagine the lack of interconference play is a large part of it... I'd also suggest the massively shortened schedule is another aspect.

Those two things combined would skew things significantly.

At Wednesday, October 30, 2013 2:10:00 AM, Blogger Phil Birnbaum said...


A few commenters, on Twitter and elsewhere, noted that the Leafs seemed to score a lot of goals on the rush. One commenter called the Leafs a "counterpunching team," which I interpret to mean that their opponents may control the play more often, but the Leafs are able to sit back, ready to take advantage of their mistakes.

Would that be a more plausible description than "defend and counter?"


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