





I’ve been lurking around Pensburgh since 2013 and have watched advanced stats (also sometimes called "fancy" stats) slowly become more and more common and prominent in the discussions that take place here regarding the team and its players.

I think the references to, and discussions about, those advanced stats unquestionably reached a fever pitch during the 2016-2017 season and subsequent Stanley Cup run. Many commenters here discussed the merits of different advanced metrics and how they should be valued in light of the team’s success. This was compounded by playing Nashville in the Final, as they had their own vocal group of fans who referenced advanced stats as a way of trying to understand (and perhaps explain away) their team’s losses.

But for those fans who have never taken an interest in advanced stats, discussions about corsi, fenwick, PDO, and regression could be confusing or even nonsensical. Some Pensburgh posters had asked about a possible post to explain and delve into some of these topics. And so, this primer is designed to introduce such fans to some of the more commonly referenced advanced stats and what they can be used for.

To be clear, I do not do any original stat work on my own. I do, however, read as much as I can about advanced stats in hockey and try to make use of the various resources that are out there when forming my own judgments about players and teams. So, please don’t take mine as the final word of authority on any of these subjects. If any of this stuff piques your interest, I encourage you to keep reading and learning on your own. I think the best hockey fans are educated hockey fans!

Corsi

What It Is

Corsi is the total of all of a team’s shot attempts, including those that are blocked or miss the net. It is named after former St. Louis Blues goalie coach, Jim Corsi, who realized that a goalie doesn’t have to work only on shots that make it to the net, but he also has to react and move on any shot attempt to be ready to make a save. As a result, Corsi (the man, not the stat) started tracking all shot attempts from opposing teams to see how much work his goalies were really doing each game. Some stats geeks picked up on this and started using it for their own purposes.

How It Can Be Used

Corsi, the stat, is used as a proxy for puck possession. Since the league no longer tracks Time on Attack as an official stat, corsi can give us a reasonable feel for how much a team has the puck in a game since the team sending more pucks toward the net (even if they are blocked or miss) probably has the puck more and is controlling the flow of play. As a result, corsi is most often expressed as a percentage, in the shorthand: CF% (corsi for percentage) to show how good a team or player is at controlling play.

Generally speaking, the team that controls play better is more likely to win. But in hockey, the phrase "more likely to win" is extremely relative. Teams control play and lose every single night in the NHL because of a variety of factors. And this is the really important part: teams don't win or lose games because of corsi. They win or lose games because they don't score or prevent enough goals. Those two things are related, but not the same. Thus, as a predictive tool, corsi is extremely limited in its effectiveness. In a head-to-head matchup, the team with the better CF% might only realistically be favored to win by a 55/45 margin. That’s really not much and could very likely have no bearing on the outcome of a single game or even a seven game series.

However, there is an undeniable correlation between puck possession and winning. The Penguins, Kings, and Blackhawks teams that won 7 Stanley Cups from 2009 to 2016 ranked 17th, first, first, second, first, fifth and third in 5-on-5 score-, zone- and venue-adjusted Corsi in the seven seasons in question.

But any team can control play on a given night, so it’s much more effective to look at corsi in large sample sizes. Looking at how they control play over the course of several weeks, months, or even an entire season can give a true sense of how good a team is. In the end, over a large enough sample size, peaks and valleys will even out and every team will ultimately play to its identity.

The same is true for players: a large sample size will help smooth out peaks and valleys in their performance and we can get a better sense of how the team controls play when they are on the ice.

That being said, it’s not uncommon to see references to corsi in single-game evaluations of a player. Goals are a fairly rare thing in the sport and are even rarer for some players based on their position or role on the team. So, looking at possession numbers when a player was on the ice can give us a sense of what kind of game they had independent of whether or not they were involved in any scoring. It certainly doesn’t tell the whole story and a one game sample should never been used to make definitive judgments about any player, but it can be useful in its own way.

Ultimately, corsi is currently the bedrock of hockey analytics. But it has limitations. So be careful about going on hockey blogs and ranting about how your team is going to win the series because they had a 65% corsi in Game 1 and only lost because the other team got lucky/got bounces/hot goalie/bad goalie/whatever. It worked out for the Penguins in the Conference Finals in 2016. It did not for the Predators in the Cup Final in 2017.

Fenwick

What It Is

Fenwick is just like corsi, but it excludes blocked shot attempts. So a team’s fenwick numbers are just shots + missed shots.

What It’s Used For

Also like corsi, fenwick is used as a proxy for puck possession. Some people prefer it because they think including blocked shots could unnecessarily skew the numbers for a team. To wit, think about how many times a game a team shoots a puck right into a defender standing two feet in front of them. Should a team really get "credit" for that attempt when any scrub can just blast pucks into guys’ shin pads all game?

So fenwick can show possession with some accounting for team "defense": how good a team is at blocking shots or forcing the opponent to work harder for shots with an actual chance of getting to the net.

PDO

What It Is

PDO is the sum of a team’s shooting percentage and save percentage (SH% + SV%). It is most commonly referenced for 5v5 play.

What It’s Used For

PDO is essentially a measure of a team’s luck. And this is where things get tricky. To quote an article on the Winnipeg Jets’ blog, Arctic Ice Hockey:

This seems to be one of the most controversial and infuriating topics in hockey, and I've been advised to call it something else, like "random chance" or "random variation" that people with a reflexive hostility to the role of luck in sports will find more palatable. But "luck" is exactly the right word for what happens - when a bouncing puck goes over a player's stick or a deflected puck goes right in the top corner of the net - I think everyone who pays attention to hockey knows that luck drives outcomes. All we disagree on is the extent to which luck is responsible for those outcomes.

I would actually disagree with that last point. I have seen people on this site rage at the notion that luck has anything to do with sport. And it’s easy to understand why. We want to believe that our athletes, our heroes, are completely in control of their destinies and that some combination of will and skill will always rule the day.

But that’s simply not true. Hockey is a sport played by men on skates holding sticks chasing a piece of rubber around an imperfect ice surface. "Random variation" is most definitely going to be involved.

PDO is a measurement of a team’s luck because of the nature of shooting and save percentage in the NHL. For a team, the average on-ice 5v5 shooting percentage is around 8%. And the average on-ice 5v5 save percentage is around 92%. Those obviously add up to 100 and that is the average PDO across the entire league in any given season. (If you need more, we can talk about how every shot can only have one of two outcomes and how that results in a 100 league-wide PDO, but that might be a discussion for another time).

What does that have to do with luck? Well, if a team maintains a PDO well over 100 for an extended stretch of games, that probably means some player or players on the team are playing well above their averages. A goalie may be playing out of his mind or a shooter (or shooters) may be on a particularly hot streak. The inverse can also be true. If a team has a PDO well below 100 for a period of games, the goalie could be struggling or the shooters could be snake bit. Because these things are outside of the established norms, and we don’t know when they will start or end or how long they will last, we call it "luck", good or bad. Or "random variation", if you prefer.

Sure, some teams can more reasonably stay above the 100 mark because of the makeup of their teams. The Penguins with their elite offensive talent or the Rangers or Capitals with their goaltending can maintain PDOs in the 101-102 range without it really being "luck". But if you see a team at 103 or above for more than a few games at a time, you had better believe that team is getting some luck and that it’s inevitably going to come back down.

Let’s look at some examples.

This past season, the Columbus Blue Jackets posted a historic 16 game winning streak that pushed them toward the top of the league standings. During that stretch, they had a 104.1 PDO.

In 2015-16, the Florida Panthers posted a 12 game winning streak that had many pundits pegging them as Stanley Cup favorites. They had an absurd 107 PDO during that stretch.

Also this past year, the New York Rangers and Montreal Canadiens both got off to scorching hot starts to the season, going 12-4 and 13-2-1 respectively in the first 16 games. And both did so on the backs of unsustainable PDOs. Montreal posted a 104.6 thanks in large part to a 95.16 SV% because… you know… Carey Price. And the Rangers had a slightly better 105.0 PDO. But their goaltending was fairly average at 92.65 SV%. Their shooters, on the other hand, seemingly couldn’t miss, posting an even-strength shooting percentage of 12.32 (for reference, the league average SH% on the powerplay was 12.76 for the season) .

They were filling the net at an historic rate and some were declaring them Stanley Cup favorites. Had they assembled the greatest collection of shooters in the history of the game? Had they unlocked the secret to solve every NHL defense and goaltender? No. They were getting lucky. And anyone who pays attention to this sort of thing knew it was only a matter of time before things cooled off, which, of course, they did. The Rangers ended up finishing 7th in 5v5 goals for, slid to the 2nd Wild Card spot, and lost to Ottawa in the 2nd round.

I don’t mean this to sound like I’m reveling in the failures of these teams as being lucky but unskilled (I mean, I kind of am, but who hasn’t?) because the truth is sometimes you need a little luck. The Penguins won the Stanley Cup last year with a perfectly average 99.9 PDO. They controlled play wonderfully and eventually just wore teams and goalies down through sheer volume. This year the team was obviously not the same and they won the Cup with a 103.0 PDO. They got "lucky". That luck could have run out at any time, but it lasted just long enough to carry them all the way.

Last year, the Sharks led the playoffs with a 103.5 PDO. It got them all the way to the Final but still wasn't enough.

In my work playoff pool this year, I picked the Minnesota Wild to win it all. Despite a rough end to the regular season, I thought their underlying numbers looked good. Then they scored 4 5v5 goals against St. Louis, shot an abysmal 2.72%, had an equally bad 95.5 PDO, and were done in five games, underlying numbers be damned. They dominated play against St. Louis. They just got horribly unlucky, or maybe I should say, "Their random variation happened at the worst possible time".

Further Reading

This has been a lot of fun to put together and I hope people find it informative. As mentioned above, I’m far from an authority on these subjects, so further discussion in the comments is more than welcome.

If the interest is there, I’d be happy to continue on with additional installments.

In the meantime, here are some great resources to check out.

naturalstattrick.com and puckalytics.com provide advanced stat tracking for players and teams. Corsica.hockey was my favorite, but is currently down for an overhaul. The owner is hoping to have it back for the upcoming season.

http://ownthepuck.blogspot.com/ has HERO charts, fantastic visual tools for comparing and evaluating players.

Dom Luszczyszyn’s work with his Game Score metric, which uses both traditional and advanced stats, can be found at https://gamescorecharts.wordpress.com/

Finally, Twitter is a hotbed of hockey analytics. If you’re on there, I would say @domluszczyszyn, @SteveBurtch, @IneffectiveMath, and @DTMAboutHeart are must-follows.