Light Work: What exactly is PDO, and what does it have to do with Dallas?
Why understanding this ancient "fancy" stat is useful.
Part of what I enjoy about fancy stats and analytics — or what I just like to call information — is that every now and then you get to expose lies. In this case, we’re talking about the lie of what it means to ‘earn a victory.’
Did Dallas earn last night’s victory versus Calgary, in which they were out shot 52-107 (attempts), 20-48 (on net), and 14-25 (high danger)? “Well, two points is two points.” That’s not what I asked. “A win’s a win.” But is it sustainable winning?
This is really the heart of what “PDO” is. Like Corsi, PDO isn’t even an acronym. It’s just the online handle of the person who invented it. Often referred to as a the ‘luck’ stat, if PDO is good for anything, it’s exposing the lie that all wins are created equal; that two points are two points; that every win is manifest destiny.
So what is it? PDO refers to the combination of shooting percentage, and save percentage. This number should typically add up to 100 percent over a long enough timeline. For example, if there are two goals in a game on 40 shots, then five percent of the shots went in. If 38 saves were made on those 40 shots, then goaltending saved at a rate of 95 percent. 95 plus 5 = 100.
The idea behind the math here is that we get a glimpse—although by no means a study—of fortune. Fortune is, I think, a better word here than luck, in part because it gives respect to the shooters and netminders who consistently defy the hockey gods. A high PDO means a team is very fortunate because it means they have both fortunate scoring, and fortunate goaltending. High PDOs are not a death knell for any team. But they are worth your attention. Last season, the top five PDO teams at even-strength were Vancouver, Boston, Winnipeg, Montreal, and Arizona. Not exactly a group of ninja turtles.
As you can see by this handy graph from Christian Lee, PDO is often erratic early on. And then it…regresses to the mean.
So what do fortunes hold for Dallas?
Last season, it was a literal roller coaster. Their fortunes had ups and downs thanks in part to Jake Oettinger’s uneven season, especially at even-strength.
Conversely, Oettinger and Casey DeSmith have been pretty darn good this year. In fact, their stability has kept Dallas above the mean all season, culminating in another high point over this recent stretch of games.
In other words, the Stars are one of the best teams in the league right now because they’re talented, and they’ve been fortunate. But that’s not what we’re interested in. We’re interested in what it means long term. What we can expect based on this information? Is Dallas due for ‘regression’?
Regression doesn’t mean losing after winning a lot. Nor does it mean winning after losing a lot. It simply means the bounces tend to even out. Dallas’ goaltending is sustainable. The shooters they have can sustainably be extra fortunate given their talent level. But can these two align if everything else is not?
That’s the rub. Yes, it’s better to be lucky than good. But ideally you possess a little bit of both — especially if you want to Call yourself a Cup contender. Dallas hasn’t had that lately.
Can that change? Well, we know who can help change that, hence my thesis at D Magazine. But Dallas needs to find a way to change without him because Miro Heiskanen’s return is not guaranteed when the playoffs start.
This is not a call for doom and gloom. In fact, the top five PDO teams this season are: (Descending) Washington, Winnipeg, Tampa Bay, Dallas, and St. Louis. Those are all really good teams1. Dallas is a really good team. So the point here is not to sound any alarm bells here. Well, maybe. But that’s not the point. The point is that Dallas is kind of winning with fortune more than merit, at least for now. Can that change? Absolutely. I would expect it to change just off the strength of the roster. That’s what makes a win like last night over Calgary encouraging in some ways. Dallas doesn’t need to be perfect. In fact, they can be quite dreadful and still pull a victory rabbit out of the hat.
However, in the playoffs, you can only steal so many games. And you have a limited amount of games in which you can steal them.
TL;DR It’s usually best not to resort to stealing2.
Well, St. Louis is its own story but still.
Shoutout to everyone on the Discord who prompted this discussion (and yes, the link is your cordial invitation).
David, I am laughing to myself about your article because you are saying exactly what I told my wife last night. She asked how the other team could keep the little black thing so much more than the Stars but the Stars are still winning. I said it is sheer talent. I said for every 10 times the other team shoot the Stars only needed 1 rush to score. The Stars have played so poorly without the puck the last half dozen games you would think they are a team on the outside looking in. But when you have 2 guys that have more goals than anyone in the league since the end of January you still win games.
Teams our scouting our weaknesses and WILL get better a exploiting them… especially more talented teams -in the playoffs… unless the Stars make significant changes to address those weaknesses.