Let’s Talk About Regression


The Calgary Flames play their 20th game of the 2014-15 season tonight – preview coming later on today – and the hockey newswires are abuzz with discussion of the Flames red-hot start…and the inevitability of their collapse down the stretch due to one thing.

Statistical regression.

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At the risk of sounding like an apologist for the team, let’s talk about regression.

First off, let’s get this out of the way: the Flames crashing down to Earth this season is not in-and-of-itself an inevitability, though I personally think there’s no damn way they can keep up this pace.

The thought process behind “Flames are gonna regress!” is partially based upon Calgary being among the NHL’s worst teams in terms of Corsi Close at 46.0% – 26th in the NHL and well below Vancouver’s 50.9% league median score. Bad possession teams don’t win a lot of games, so the fact that the Flames are winning a lot of games despite not being a bad possession team suggests something funky is going on. And the data seems to bear that out: the Flames have a PDO of 102.8 right now based on a 92.62 save percentage and a 10.12 shooting percentage. In short: a lot of the Flames’ success right now is based on proverbial smoke and mirrors.

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*Need help with these stats terms? Click here to learn about Corsi and PDO

Let’s break that down a bit.

The save percentage is more or less league average; the median team in the NHL right now (Boston) has a 92.07 save percentage (and last year’s median was 92.26%), so the Flames aren’t that far above average. And based on last year’s horrid 90.89 number, they were probably bound for a course correction.

As for the shooting percentage? Let’s not mince words, 10.12% isn’t sustainable over an 82 game schedule. Last season’s best team (Anaheim) had a 9.83%. Two seasons ago, Toronto was able to maintain a 10.57% over a 48-game schedule and we all know how that turned out. Looking back at several years of team performances, it’s exceptionally rare that a team can maintain a shooting percentage north of 9 over a full season. Most likely the Flames crash down towards the mid-8% range, if not a tad lower even.

So why is it so high now?

Two main factors: luck and shot location. The first is easy: pucks are bouncing Calgary’s way when they could’ve bounced otherwise. The second?

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Via our pals at Sporting Charts, here’s a comparison of shot locations from this season and last season.

Screen Shot 2014-11-17 at 10.42.38 AM

Calgary is a bit more effective at driving chances towards the front of the net, something that’s generally been born out of our own scoring chance data. Last year was a lot of perimeter play, this year involves more chip-in plays that utilize team speed to get past defenders. As a result, they’re getting more shots in high-percentage areas and more pucks are going in during these chances.

Can it work over an 82 game season? We’ll see. The California teams of the Pacific Division offer a nice test, and the change in strategy in the summer may have been a result of the Flames getting knocked around a bit by big, mean Kings, Sharks and Ducks defenders. If you can’t beat them with size, beat them with speed.

So far, the Flames have been.

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So when IS regression coming, if it’s inevitable? Well, that’s the thing – nobody knows.

Here’s a quick table to illustrate the point.

Season(s) #1 PDO #30 PDO
2013-14 102.5 (BOS) 98.0 (FLA)
2012-14 101.9 (ANA) 97.3 (FLA)
2011-14 101.6 (BOS) 98.1 (FLA)
2010-14 101.9 (BOS) 98.4 (FLA)
2009-14 101.3 (BOS) 98.6 (NYI)
2008-14 101.6 (BOS) 98.6 (NYI)
2007-14 101.4 (BOS) 98.6 (NYI)

Regression doesn’t operate on a set schedule. Florida didn’t get appreciably “luckier” until FOUR YEARS of data was accumulated – or roughly 294 games, while it took two seasons for Boston to transition from “Calgary Lucky” to just fairly above average. Look at how Boston’s numbers stuck around as the sample grew from three years to seven.

For the curious, the distribution of PDO got more concentrated around the theoretical “100.0” mean value as the sample got bigger, but as you can see from this example, it’s a distribution with some long tails, even though the theory behind the data suggests everyone should crash down to 100.0 after infinite games.

Oh, and what quality does Boston have over this sample size that could make their PDO numbers a bit more resilient? They’re a great possession team, with a seven-year Corsi Close of 52.7% (fourth in the NHL in that span). With the Islanders and Panthers in the bottom-third of the NHL in that statistic, that could explain their lousy numbers.

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Can Calgary have a sky-high PDO all season? The numbers say that it’s not impossible but it’s pretty unlikely, particularly given that the difference-maker for them has been insane shooting team-wide – high save percentages have tended to be a bit more resilient historically.

It’s not that the statistics community is trying to rain on anybody’s parade; it’s just that when you see some rain coming, it’s merely common courtesy to suggest everyone grab an umbrella. We’re not sure when the rain is gonna get here, but sooner or later, the rain – and Calgary’s PDO – is gonna fall.

  • smith

    There’s nothing that says PDO needs to regress this season.

    Therefore, I say that the Flames’ PDO will regress next year. For this year, they will make the playoffs and win the Stanley Cup.

  • Koolmoedee

    One thing I’ve been curious about. People talk about Corsi like it’s an all-revealing stat. However, I rarely hear about shot blocking being taken into consideration.

    Shot blocking has become a far bigger part of the game, especially for some teams like Calgary. In fact, the Flames are second in the league in shot blocks.

    For sure, it would be better if the Flames allowed fewer shot attempts against, but if they are blocking shots at a higher rate than other teams those additional shots are not turning into goals.

    Indeed, the Flames are right in the middle of the league – 15th – in shots against per game.

  • RKD

    Good article. the most compelling data was the large sample PDOs showing Boston at the top for a long time and FLA/NYI at the bottom for a long time.

    First thing that comes to mind on those teams is great goaltending (Thomas/Rask) and pretty bad tending on the Isles and Panthers.

    If you had a very good goaltender for infinity games, could you sustain above 100 PDO the whole time?

  • slapshot444

    Whats not being measured are things like; team, enthusiasm, spirt, belief,
    leadership and youth. Try telling a young person they don’t know what they don’t know. They can still charge head first in the world of anything and be successful powered merely by enthusiasm, and you know what, it usually works. A team of any sport can rise above the statistical sum of its parts
    when the intangibles come into play. After that the success continues by the experience of continued wins.
    Mostly sport is governed by intangibles and there fore impossible to predict no matter what stats you have on hand. Are we on the road to the cup? Likely not. We are in the top five in the league in the standings though, and you can’t take that away.

  • Kevin R

    A little ranty but I just wanna get this off my chest…

    I’m enjoying this while it lasts. I’m hoping it won’t end anytime soon because its too damn fun! We’re watching Johnny Gaudreau live up to all of the hype. We’re watching one of the best defensemen we’ve seen in the Saddledome EVER having a truly remarkable run! I’m watching one of the best starts to a season in my favourite team’s history and while all this is going on I have WAY too many people telling me to chill with all that because: THE END IS NEAR! I just wanna enjoy this. Can’t we wait for the eventual collapse before we all start piling on.

    I feel like the issue that bothers me when it comes to discussions of PDO is that it measures something that is, mostly, out of anyones control. Of course every player tries to score on a majority of there scoring opportunities. A coach doesn’t turn to his goalie and say, “I want you to go out there and stop a large percentage of the shots you face tonight!” …Yeah… That’s just his job.

    My question is this: What should the organization do about it? What would you do about it? How does a team manage it’s own ‘luck.’ I don’t think that’s possible.

    I think the coaching staff and the players have a responsibility to, if not be a great NHL team, at least be one that works it’s asses off and this team does that. Better than almost any team in the league. What is there to argue about that and in what way are the flames not living up to your expectation as a hockey fan?

    I like ALOT about what new analytics can show me under the surface of a hockey game. I like the way different analytics both challenge and affirm my beliefs when it comes to the way the game should be played. Despite all this though, the thing that excites me the most about hockey analytics (and sports analytics in general) isn’t it’s predictive nature but rather the things it CAN’T predict. The Outlier. That’s why we watch sports to begin with; to see people overcome their expectations. I’m watching a Flames team do just that and I’m giving them all the credit in the world for it.

  • On lowetide’s Oiler Blog, G Money, Younger Oil, and I were toying with the idea of PDORel to see how likely CGY was to regress(I know, all us Oiler fans care about is CGY failing, god I hate the oilers).
    The ideas based on TSN panelists arguing that if you have a team built around consistently solid goaltending and consistently solid snipers, your PDO could stay above 100 for years.

    What we did was take the overall average career shooting% of all the players on CGY(except rookies, for them we used the average shooting% of players in the same draft range) and we grabbed the career avg sv% of the goalies and applied a formula where 0 would be the ‘expected’ result.

    (Current sh% – career sh%)+(current sv%-careersv%)

    It was posted in the Oilers vs Vancouver blog two day ago.

    The result was something like +1.12% above 0. Which was less than one standard deviation away from the median. So based on that, and CGY’s total shots for and against, their overall goal differential isn’t actually that far above what the careers of its players and goalies should expect exp

    I’d have to go back to find the exact result.

    Oilers however, turned out to be the unluckiest team in the league by this measure.

  • “These stats suggest something negative about my favourite team! Therefore I shall choose to disbelieve their value despite it having been demonstrated over and over again!”

    Just… dumb.

    Amazing what people are willing to convince themselves of when it comes to their hockey teams. We see it every time – Toronto in the lockout year, Minnesota when they were beating the league. Just backlash when being told “it’s not going to last and here are the objectively verifiable reasons why”.

    You can enjoy it in the meantime. You should. But be realistic about who the team is. They’re not as good as their record indicates. This should at minimum give you some comfort when they do come back to Earth – it’s not a disappointment; that’s just who they were all along.

    And hey, let’s also note that even though they’re not as good as their record, they’re still better than the Oilers and Leafs.