What Lightning fans should know about PDO

What in the world is PDO? And why should Lightning fans care? Well, given the disappointments fans have suffered this season, this statistic actually might be a sign that things could possibly get better. So, I'm all for it.

Numbers are taking over hockey, if you haven't noticed. I've recently done a lot of posts using numbers and that's quite remarkable. I hate math. I hate trying to add up grades at the end of a semester. The most advanced function my calculator performs is the square root. Which I've never used in any practical application.

But these adventures in statistics are a good way to check your perceptions against reality. Because observation is flawed. Our brains trick us into thinking we're seeing things we're not, and as human beings, we're very, very good at discovering patterns--even when those patterns aren't there. Over generalization is a great example of that phenomenon. A player turns the puck over and the fans start to say, "That guy is always turning the puck over." The next time it happens, you notice it, because your brain is looking for a pattern. Statistical analysis aims to remove some of that and allow for a less biased understanding of what's going on out there.

But the statz nerdz have long insisted that they don't understand everything, that they can't predict outcomes in individual instances or in the short-term, and that there's a great deal of apparent randomness involved in hockey. The rest of us may be thinking, "yeah, I've noticed." The numbers guys and gals are thinking, "I wonder how we can account for that."

And that's what PDO is intended to try to do--account for "luck." Here's the premise: Whether a specific shot on net results in a goal is completely unpredictable. It's almost all luck, or to be more accurate, randomness. There are two measures of whether shots on goal result in goals, shooting percentage and save percentage. For any game, one team's shooting percentage plus the other team's save percentage always adds up to 1 (or 100%.) But sometimes a particular shot gets deflected or the puck is wobbling or the goalie gets a stick on it or something else keeps it out or sends it in. That sort of thing is, statistically speaking, random--it's unpredictable and unrepeatable.

Long story short, for reasons my puny brain doesn't quite comprehend, over the course of a season any team's shooting percentage plus that team's save percentage regress quite strongly toward (that is, tend to move closer and closer to) 100%. (PDO is sometimes expressed as percentage, sometimes as a decimal and sometimes as something around 1000. They all seem to mean the same thing, though.) That regression toward 100% makes this a nifty little measure of whether current performance is sustainable or the result of randomness. You know. Luck.

A team with a low PDO will have a pretty strong tendency toward getting better. A team with a high PDO will have a pretty strong tendency toward falling off. The farther away from 100% your team is, the more likely that change in luck will occur.

The Lightning, as of Sunday afternoon, have a 5-on-5 save percentage of .902 (90.2%) and a team 5-on-5 shooting percentage of .084 (8.4%). Add those together and you get .986. So how low is that, really? Well, it's the 6th lowest in the league right now. According to my figures, the league PDOs range from .971 to 1.037 (the Colorado Avalanche with the lowest and the New York Rangers with the highest.) So, while PDO doesn't tell us anything about what particular areas are most likely hurting team performance or propose any solutions, it does tell us that randomness--those ubiquitous "bad bounces" everyone talks about--have had something to do with the outcome of the team's games. It also reminds us that those bad bounces are unlikely to be sustained over the course of the season.

So, while there's a lot I don't quite understand about PDO, what I do know tells me that there's hope that things will get better. If only I could say when it would happen and how much better it might get....

Thanks to Dirk Hoag at at On the Forecheck for helping me find the data I needed to be able to do this article. You can get it at www.behindthenet.ca/2011/team_data3.php . For more info on PDO, I'd start with the article "PDO: If you were going to understand just one NHL statistic," at Arctic Ice Hockey.Or try "PDO And What It Means" at PensBurgh.com. Also good are "Advanced Team Stats - What the Heck is PDO?" from Pension Plan Puppets and "PDO Numbers" from Tyler Dellow. A lot of the information out there on PDO relates to individual players rather than entire teams, but a Google search for "PDO hockey" brings up a lot of articles from good sources like Arctic Ice Hockey (which used to be the original Behind the Net) and mc79hockey. Look around for it if you like, and maybe you'll see stuff I've missed.