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The Eye-Test Vs. Corsi: Canucks Game 1


JamesB

Eye-Test vs. Advanced Stats  

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People overthink advances stats. Corsi is just a way to try and measure possession time, since a team shooting the puck a lot obviously has it a lot. Of course, that doesn't measure quality of possession (or quality of shots) so you have to consider that in context of other stats and actually viewing the play.

 

I think of it this way: if you couldn't see the game and didn't have advanced metrics you'd have an even worse view of what actually happened. Advanced stats may be misleading if taken on their own, but it's a way to see more of what was happening (i.e. the Canucks spent a fair bit of time defending against a strong offensive team, and Horvat and Sutter's lines did the lions share of work in that while the Sedins were given easier opposition and deployment). Doesn't tell you explicitly that the Canucks won the game, but it's something.

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1 hour ago, elvis15 said:

People overthink advances stats. Corsi is just a way to try and measure possession time, since a team shooting the puck a lot obviously has it a lot.

No, it's actually a shot-differential metric that is misnamed "possession".  Taking lots of shots does not necessarily correlate to a team or line that "obviously has it a lot."

 

The Sedins have the puck a great deal more than they shoot the puck - anyone that has watched them over the years realizes that they make multiples more passes than most players and literally possess the puck far more, while taking fewer shots relative to most players.  They have had high 'possession' numbers nevertheless (high shot differentials in their favour) - but their actual possession was far more dominant than those metrics indicate.

 

Other guys, particularly players that lack vision, fire the puck first and look to pass as an afterthought (generally bass ackwards).  Their shot attempt differential can be misleading in a reverse sense.  It may be generally true that it never hurts to simply get pucks on net - however, when you're facing a great  - or hot - goaltender, that theory grows thin in that context - you either need to create traffic, or actually create.

 

But in any event, corsi doesn't measure possession time - it's far too lazy to do that - actual possession time is not measured (and wouldn't really be that difficult).

It's easier to track possession time in a game like football where it rarely changes in the midst of a play, and can easily be tracked by time stoppages - but the NHL doesn't really track possession - and 'advanced stats' really should stop referring to shot differential metrics as "possession" - which it simply is not.

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I think Corsi is a non-factor in evaluating the game unless you're right there evaluating the team on ice in person, and you're looking for an extra edge. For me it is totally useless as a regular fan of the game.  Most people attempt to extrapolate too much from the completely non-representative numbers.

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6 hours ago, oldnews said:

Depends on the eyes though.

 

A lot of 'eye tests' depend on eyes that need to be tested / a set of glasses and/or the crap wiped out of them lol.

 

3 hours ago, oldnews said:

No, it's actually a shot-differential metric that is misnamed "possession".  Taking lots of shots does not necessarily correlate to a team or line that "obviously has it a lot."

 

The Sedins have the puck a great deal more than they shoot the puck  - anyone that has watched them over the years realizes that they make multiples more passes than most players and literally possess the puck far more, while taking fewer shots relative to most players.  They have had high 'possession' numbers nevertheless (high shot differentials in their favour) - but their actual possession was far more dominant than those metrics indicate.

 

Other guys, particularly players that lack vision, fire the puck first and look to pass as an afterthought (generally bass ackwards).  Their shot attempt differential can be misleading in a reverse sense.  It may be generally true that it never hurts to simply get pucks on net - however, when you're facing a great  - or hot - goaltender, that theory grows thin in that context - you either need to create traffic, or actually create.

 

But in any event, corsi doesn't measure possession time - it's far too lazy to do that - actual possession time is not measured (and wouldn't really be that difficult).

It's easier to track possession time in a game like football where it rarely changes in the midst of a play, and can easily be tracked by time stoppages - but the NHL doesn't really track possession - and 'advanced stats' really should stop referring to shot differential metrics as "possession" - which it simply is not.

 

8 hours ago, Jimmy McGill said:

@JamesB nice evaluation. 

 

I've been perusing the analytics sites for about a year now, and i do have an R&D background that includes stats training. So what I see lines up with what you are describing here, a significant over-emphasis on corsi because its easy to measure, but also a lot of NHL fans tend to focus on scoring much more than anything else (just my 2cents but that seems to be the overriding emphasis in the media as well).

 

The big failure of hockey analysts imo is a lack of ability to describe a game like Saturday. Where i'd add to your post is zone starts. By looking at that and corsi we should have been brutalized by the Oilers. So.... whats missing from his picture? No ability yet to adequately describe two things: checking lines, and pure defensive play. I haven't seen anything really to describe that well at all, so we're left with noobs like Travis Yost telling us Gudbranson is a hot turd but anyone that watched the game knows how important he was in the win. 

 

There needs to be a statistical analysis that can look at pure defensive play before we can really start to take hockey analytics seriously, otherwise its not really more insightful than basic stats.  

Some very good points here from @oldnews and @Jimmy McGill.

 

Yes, there are a couple of major problems with the eye test.

First, some people just don't have a very good eye. Most people on CDC have probably watched enough hockey and many have played enough that they have a pretty good eye for the game. But most fans see the obvious things -- goals, fights, big hits, big saves, but miss a lot of other stuff. 

Second, and more important (as has often been mentioned on CDC), most us have a tendency to see what we want to see or what we expect to see (confirmation bias). That is particularly true with players like Sutter and Gudbranson, whose names have come up in this thread. Their strong points (defensive play for both, physical game for Guddy) are not easily captured by numbers but supporters will see those plays. Critics will see their negative qualities, like poor passing and puck handling.

 

The points about CORSI not being a possession stat and about perverse effects (lower skill players might actually tend to shoot more instead of setting up the more subtle but higher percentage play) are also good ones. There are some actual possession stats done on small samples -- actually measuring time spent controlling the puck, spent in the o-zone, etc. Unfortunately I cannot put my hands on a URL here. I can think of quite a few college or university students who would be happy to watch film and track possession time for $15 an hour or so, it would not be hard for the NHL to do. 

 

Zone starts are also important. I have tried to think of how to adjust CORSI for zone starts. There must be a good way to do it, based on estimation of the normal relationship between zone starts and CORSI. For example, an ozone start probably leads to an expected value of maybe one or two shot attempts within the next 30 seconds, whereas a dzone start maybe has an expected value one or two shots against and maybe only 0.5 shots attempts for, or something like that. Then it would be possible to check a player's CORSI relative to expected CORSI. I would think about this more if I thought CORSI itself was a useful measure.

 

And I agree that pure defensive stats would be nice to see. There are few things, like shots allowed per minute, etc. but quality of opposition is so crucial for this stat that without dealing with that there is not much point.

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what are you measuring and how well your tools measure that is an important part of stats - as you note.  Of course you have a small sample and I don't believe any data that cannot be checked for significance.   I would also say that the eye ball is good for a one game assessment and that the stats are more suited trying to make sense of 20 plus game sample.  Watching the game the other night I wasn't caught up in the hoopla (loved the win) and my 'eye' tended to see more of the trends in the corsi.    In other words we won that game but if we played the oilers the next 10 they would beat us a majority of the time because are top lines have some flaws that got covered up in one game.  That is where I would say the corsi would be more useful and accurate or at least as good as a good eye ball.     

 

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51 minutes ago, JamesB said:

 

And I agree that pure defensive stats would be nice to see. There are few things, like shots allowed per minute, etc. but quality of opposition is so crucial for this stat that without dealing with that there is not much point.

I think we're gong to have to see some sort of machine vision system that can track when guys like McDavid are annoyed by guys like Dorsett :lol: but I'm half-joking, we're going to need some kind of very detailed and objective data source to find correlations... or we can just enjoy the game too. 

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33 minutes ago, Jimmy McGill said:

I think we're gong to have to see some sort of machine vision system that can track when guys like McDavid are annoyed by guys like Dorsett :lol: but I'm half-joking, we're going to need some kind of very detailed and objective data source to find correlations... or we can just enjoy the game too. 

Radical concept. Don't think it will catch on.

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10 hours ago, JamesB said:

The Canuck game 1 provides a very good illustration of the contrast between using the "eye-test" to evaluate player performance and using advanced stats, particular shot metrics such as CORSI and FENWICK. Personally, I like some advanced stats but I do not like these shot metrics. Game 1 against Edmonton illustrates the problem. First, for anyone interested, the shot metrics are available here: https://www.hockey-reference.com/teams/VAN/2018.html#all_stats_adv_rs

 

The eye-test summary of the game runs as follows.

 

1. First, Travis Green,  the reporters (even Botchford), the players, and most of CDC thought that the Canucks played a good game against a good team. After all, the Canucks won the game and a lot of guys looked good doing it.

2. The first star of the game was Horvat, who scored two impressive goals and looked great -- fast, strong, high intensity.

3. The "second star" of the game was the Sutter line (with Dorsett and Granlund) who shut down arguably the best line in the NHL (with McDavid and Draisaitl) and scored a goal.

4. Markstrom let in a questionable first goal but was otherwise very good.

5. If there was a disappointment in the game for the Canucks it was probably the Sedin line (with Vanek) who looked slow out there and did not do much on the PP despite lots of PP time.

 

The CORSI (shot attempt) summary is completely different and illustrates the weakness of CORSI numbers. Here is the even strength CORSI summary.

 

1. The Canucks had a bad game, getting crushed by Edmonton on shot metrics.

2. The best Canuck line by far was the Sedin line. The top 3 forwards in CORSI were Daniel (61.5%), Vanek (57.1%), and Henrik (50%).

3. The worst Canuck line was the Sutter line. Their shot metrics were Granlund (22%), Dorsett (25%) and Sutter (26%).

4. The Horvat line was not much better: Horvat (32%), Baertschi (32%), Eriksson (33%).

 

This comparison illustrates my pet peeves about CORSI.

 

1. Most importantly, CORSI does not adjust for strength of competition, which is a huge factor. The difference between going up against the McDavid line and going up against the Edmonton bottom 6 is huge in terms of expected CORSI. The Sutter line was able to keep the McDavid line from scoring, forcing them into a lot of missed shots, blocked shots, and  low percentage shots, all of which count in CORSI. They did a great job. The Sedin line played the Edmonton bottom 6 about even -- which is not such a great accomplishment. There are some attempts to adjust for quality of competition but they are woefully inadequate. The explanation is long, but it is very hard to adjust properly for quality of competition. 

 

2. CORSI is usually reported for even strength play. But special teams are obviously very important. Factoring special teams in correctly is difficult. Once again, in this game, Sutter, Dorsett and Granlund were good on the PK. The Sedins did not play PK but they led the forwards in PP time and did not do much in that time.

 

3. Shot quality is obviously important. Generating low quality shots, missed shots, and blocked shots is obviously less valuable than driving the net (as Horvat did on his first goal) or getting close-in chances (as on Horvat's second goal). There are attempts to adjust for shot quality, but it is hard to do well. Horvat and Baertschi go not generate a lot of low quality chances but play hard to get high quality chances. This hurts their CORSI numbers.

 

The point of this post is not to criticize the Sedins. They have had great careers and I hope they have a good year this year. They were okay in the first game. And my point is not to criticize advanced stats. However, looking just at even strength CORSI is often very misleading. Getting good advanced stats is difficult. I am, for example, also not a fan of GAR (goals above replacement) or WAR (wins above replacement), largely because they fail to account for quality of competition (and quality of teammates). 

 

Stats I like include actual zone time metrics, defensive zone exit stats, and metrics based on high quality scoring chances. However, all of these should be adjusted for quality of competition and quality of teammates.

 

Any comments would be appreciated.

 

 

@JamesB this is not a good post...this is a great post.

We all see these games differently, and it makes these boards enjoyable.

Some have a better understanding of Corsi/Fenwick numbers and stats than others, but the eye test is surely the most important part.

 

You can actually go 'blind' at staring too much on analytic numbers... as well as a few other things...

 

I literally had to stop handing out pluses to responders, as I would have none left for the game tonight.

+10

 

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Corsi is often subject to score effects making it unreliable as a score based metric (Better corsi doesn't mean winning). In fact a large part of the league could actually use Corsi as a way to measure defend a lead. Teams with decent Corsi while winning probably have a better chance of holding a lead till the end of a game.

It also depends on style too. Counter attacks often produce more high quality chances, but less shots overall. Depending on the make up of your team, it may be more beneficial to have one or the other, regardless of which has higher Corsi.

Scoring chances are a pretty good stat that coaches often track, but it's very subjective. If we want to see if a team os playing better than another it would be great to look at

 

  1. High danger shots
  2. Shot Assists
  3. Multi Passing Plays that lead to a shot
  4. Passes from below the goal line

Which ever team has more of these during tie games is probably going to end up on top. Which ever team is best at limiting these while ahead is probably going to win, and which ever team is best at creating these while behind could come back.

You could also track this real time to see what your most effective line is during a game offensively and defensively in different situations

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8 hours ago, oldnews said:

No, it's actually a shot-differential metric that is misnamed "possession".  Taking lots of shots does not necessarily correlate to a team or line that "obviously has it a lot."

 

The Sedins have the puck a great deal more than they shoot the puck - anyone that has watched them over the years realizes that they make multiples more passes than most players and literally possess the puck far more, while taking fewer shots relative to most players.  They have had high 'possession' numbers nevertheless (high shot differentials in their favour) - but their actual possession was far more dominant than those metrics indicate.

 

Other guys, particularly players that lack vision, fire the puck first and look to pass as an afterthought (generally bass ackwards).  Their shot attempt differential can be misleading in a reverse sense.  It may be generally true that it never hurts to simply get pucks on net - however, when you're facing a great  - or hot - goaltender, that theory grows thin in that context - you either need to create traffic, or actually create.

 

But in any event, corsi doesn't measure possession time - it's far too lazy to do that - actual possession time is not measured (and wouldn't really be that difficult).

It's easier to track possession time in a game like football where it rarely changes in the midst of a play, and can easily be tracked by time stoppages - but the NHL doesn't really track possession - and 'advanced stats' really should stop referring to shot differential metrics as "possession" - which it simply is not.

Feel free to put me back on ignore, especially if you only want to quote one of the two sentences that relate to the topic you felt a need to correct me on.

 

The intent of Corsi was to find a way to track possession, as a team with 60 shots is thought to have the puck more than a team with 30. But - as I said - that doesn't measure the quality of possession and has to be used with other stats and actual game viewings. It doesn't take into account other variables (e.g. the Sedins cycling the puck looking for the perfect spot, or guys without vision taking shots any time they get the puck) and of course isn't a literal measure of possession time which is why I posted two sentences, not one.

 

Until someone actually does track the literal time a team/player has possession, we're left with trying to find ways to approximate it. You look at things like Corsi - certainly a step above plus/minus - and zone starts and other metrics, as well as what's actually happening in the game and what kind of player you're tracking (e.g. the Sedins who cycle vs a guy with no hockey sense) if you want more than just watching the game.

 

If it was perfect, everyone would use it. But, there are some people smart enough to do so and get reliable findings.

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6 hours ago, elvis15 said:

Feel free to put me back on ignore,

ok will do - and can't be bothered to read beyond that - if that's your irrational response to a pretty straight forward and reasonable post ie nothing personal whatsover - a simple discussion of the meaning and merit of 'analytics'.

cheers.

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17 hours ago, Shirotashi said:

The sutter line had low Corsi numbers you say? Where in those numbers does it demonstrate how they shut

down the oilers best line, 20million in salary. If a line does not score a goal but prevents 2-3 where is that shown?

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and.....
 
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On 10/09/2017 at 11:57 AM, Rick Blight said:

I will supplement the eye test with advanced stats but rely primarily on the eye test.

If I wanted to rely primarily on advanced stats I would probably become an Arizona fan and that ain't going to happen!

Or Florida.

Word is they had a team bonding session during camp where they all rolled around on some stat sheets together blindfolded.

Arizona is seriously considering for next season.

 

 

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