CORSI & SHOT QUALITY
Corsi is a very good predictor of team success. In one study, Corsi correlated with team standings points at r=0.62, which is pretty high.
So, the consensus is that if a team’s Corsi doesn’t really match their won-lost record, the difference is probably luck, and the team shouldn’t be expected to repeat.
If shooting percentage is almost all luck, the implication is that it’s not something a team controls, or can even *choose* to control. It’s like clutch hitting – just randomness that looks like there’s something real behind it.
In that case, you’d expect every team to be around the league average of 8%, in all situations. Shot quality must be about the same for all teams. Intuitively, you might think some teams are good enough to have more breakaways and blind passes, while other teams take a lot of harmless shots from the point. But, the data show otherwise.
7.60% … down 2+ goals
7.75% … down 1 goal
7.52% … tied
8.40% … up 1 goal
9.19% … up 2+ goals
Why such big differences? My guess is … when a team is behind in the game, it changes its style of play. It probably presses a little more in the other team’s zone, trying for better opportunities – which is why its percentage rises a little bit. On the other hand, when it presses more, that increases the chance of being caught behind on defense. That gives the other team more chances at odd-man rushes and breakaways. Which is why the opposition – the team that’s up 1 or 2+ goals – sees its shooting percentage rise significantly, all the way to 9.19%.
If playing that style is so beneficial, why don’t teams do it ALL THE TIME, instead of just when they’re in the lead?
The answer is: it’s not that beneficial. The higher shooting percentage is offset by the fact that the teams in the lead take fewer shots – that is, they have a lower Corsi. Here are the percentages of (Corsi) shots taken based on score:
57.0% … down 2+ goals
54.1% … down 1 goal
50.0% … tied
46.0% … up 1 goal
45.1% … down 2+ goals
The quantity and quality factors go opposite ways, and they roughly cancel each other out.
The team that is up dictates style of play: The team that's ahead has an interest in limiting scoring by wasting time. It might be worth playing a style that gives the opponent a slight advantage in expected number of future goals, if that's offset by a lower probability of getting a goal in the first place.
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.
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.
My last two posts argued that there might be an inverse relationship between shot quality and shot quantity – or, in other words, between Corsi and shooting percentage (SH%).
Most hockey analysts disagreed with me. Which is fair enough; so far, the only evidence I’ve really put forward is the negative correlation, the last two years, between Corsi percentage and shooting percentage.
Spent the last week searching for other evidence, and I think I finally found something.
If shooting percentage in tie games is just luck – or mostly just luck – you should expect no correlation between shooting percentage this year, and shooting percentage next year. And that’s pretty much what you see in the data.
Doing the same thing to predict goal percentage from goal percentage (the percentage of goals in your games that are yours), I found a stronger relationship, a correlation of 0.27.
And, as the hockey analytics community has shown, this year’s Corsi is even better for predicting next year’s goal percentage. That correlation came out to 0.40.
It turns out that every point of shooting percentage was worth 0.83 points of goals next year. So, if the league shooting percentage was 7.5%, but you had 8.5%, then, all things being equal, you should score 50.83% of goals next year.
Compare that to Corsi. Every point of Corsi was worth 0.72 points of goals next year. So, all things being equal, if this year you took 51% of shots, next year you should score 50.72% of goals.
So, the two were roughly similar, on a percentage-point-by-percentage-point basis.
Tie-score Corsi predicts the near future much better than goals. For instance, if you use the first 40 games of Corsi to predict the next 40 games of winning percentage, the correlation is .408. But if you use the first 40 games of goals, the correlation is only .312.
My argument, the last three posts, is that shooting percentage and number of shots taken are negatively related, so when the 2012-13 Leafs have a low Corsi, but a high shooting percentage, that might be an indication of evidence that they took higher quality shots.
Many hockey analysts think that’s not true. One of the reasons they give is that shooting percentage seems to be completely random. This year’s shooting percentage is no indication of what the team’s shooting percentage will be next year. If shooting percentage isn’t a skill, how can it mean anything?
I tried to explain, in my first post, how SH% can be important even if it’s random.
Instead of SH%, consider injury days lost. It’s likely injuries are pretty much random – if you have lots of injuries this year, you’ll probably revert back to the normal amount next year.
But: if a team has a low Corsi, but high injuries, the high injury rate DOES affect your estimate for next year. “Injuries are random so we shouldn’t consider them in evaluating the team’s talent” doesn’t fly.
In the past four posts, I speculated that NHL teams may vary in shooting percentage partly because they take different quality shots. I also speculated that, maybe, their shots vary in quality just randomly.
Under this model, I have to admit that real-life differences in shooting percentage aren’t due to just random differences in shot quality between teams that would otherwise be the same.
Which means: I’m forced to stick with the idea that there are differences between teams.
But is this model reasonable?
Is it plausible that teams vary in their willingness to take shots, and/or their ability to affect their opponents’ propensity to shoot? Because, that’s what this is about. If you accept that teams can differ that much in how they shoot, then you have an explanation for at least part of the Leafs’ bad Corsi.
“shot quality” issues could be a big factor – if not for all teams, then maybe at least for some of them, like, perhaps, the Toronto Maple Leafs.
As far as I know, the issue of how much shot quality impacts Corsi remains unresolved.
Here’s some more evidence I came across that seems to suggest shot quality might be a bigger issue than even I had suspected.
I ran a correlation between team scoring chance percentage and goalie save percentage. If scoring chance percentage didn’t matter, the correlation would be low. If it did matter, it would be high. (For save percentage, I used 5-on-5, tie score, both home and road.)
The correlation turned out to be … -0.44. That’s pretty high.
(One thing that’s interesting, that I want to look into, is that the SD of team quality shot percentage *for* is only about half of the SD of quality shot percentage *against* (2.7 versus 5.6). Does that mean that defenses vary more than offenses? Hmmm…)
So I think all of this comprises strong evidence that teams differ non-randomly in the quality of shots they allow. That doesn’t invalidate the hypothesis that Corsi is still a better predictor of future success than goals scored. But it *does* suggest that you can likely improve Corsi by adjusting it for shot quality. And it *does* suggest that PDO isn’t random after all.
In other words: Corsi might be misleading for teams with extreme shot quality differences.
In previous posts, I’ve argued that when it comes to shots, NHL teams might differ in how they choose to trade quantity for quality. That might partly explain why the Toronto Maple Leafs, for the past few seasons now, have had ugly-looking shot stats, but with an above-average shooting percentage.
Skeptics argue that team shooting percentage (SH%) doesn’t seem to have predictive value from season to season, which suggests it’s luck rather than skill or strategy. But, at the same time, Corsi for teams seems to have a negative correlation to SH%, which is one piece of evidence that shot quality strategy might be a real issue
If there were *never* a tradeoff between quantity and quality, every team would be shooting all the time. So, there must be some level of “dangerousness” above which a point shot is a good idea, and below which a pass is better.
CORSI PREDICTIVE VALUE AND SHOT QUALITY
Phil’s point that it’s possible for a team to deliberately adopt a low-shot high-percentage strategy in all situations is obviously true. But the only place where there is any real evidence for teams doing that is when they are protecting a lead, and even there the differences aren’t terribly large.
So if we’re going to make a case for a team doing better than shot differential would suggest, we should focus specifically on their performance with a lead, and expect that impact to be minor.
Of course, it’s possible that they’ve hit on a radical new strategy for tied and trailing situations. But since we’ve never seen that before, with each subsequent team that has a high shooting percentage in those situations, the safer bet is that they’ve simply run hot and will slide back to the pack.
Comments