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.

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.

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.

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?

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

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.

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.

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.

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.