PDO is still at the top of the league, but I continue to maintain that it’s not outlandish. Not when Rinne is having a Hart\Vezina season — James Nelson (@Predalytics) December 31, 2014

Sigh. James Nelson and I keep having this argument about the Nashville Predators’ PDO and goaltending situation. And let’s face it, he’s wrong and I’m right 😉 but we’re kinda both drawing a similar conclusion (the Preds are more good than lucky). But there’s a premise in here that’s worth talking about because even if the conclusions are similar the way we get there matters.

Okay. The Predators’ PDO has been among the highest in the league all year. It may very well end up being the highest in the league for the full season. Right now, it is just inside the normal range at 102.6. For illustration, this is how a 102.6 PDO at 36 games looks in the context of season-long PDO distributions.

Anything in the grey part of the diagram is still likely to move, but the “pressure” decreases the closer you get to the inside. Results outside the red lines are highly unusual; they are extreme outliers. The Preds are just on the line between normal and high, so there’s still room for it to come down. It’s not currently alarming by any means—or to use James’s terminology, it’s not outlandish for less than halfway through the season.

In this, James and I are in agreement. Randomness is not currently having an outsized effect on the Predators’ season and they are performing well enough to explain their success. They are not vastly outperforming their corsi, so there’s not a lot to worry about. Right now.

For a full season, 102.6 would be quite high, although teams have ended up there before. Not many, granted. Only 6 times out of 210 attempts since 2008-09, two of which happened in the 48-game season of 2012-13. But, yeah, it could happen.

What matters, though, are the causes you ascribe to it. It matters whether you think that could happen because randomness can act that way sometimes or you think that it could happen because the Predators have enough shooting and goaltending talent to beat the odds of PDO regression.

The real impact of talent on PDO is not known and it’s probably not terribly strong, particularly for teams. There have been 10 teams who have posted a PDO 102 or above since 2008-09. The Boston Bruins, who are the poster child for this kind of thing, account for three of those and Pittsburgh two others. Three of the 10 seasons were from 2012-13, when only 48 games were played. More importantly, the Boston Bruins may have 3 seasons at or above 102, but they have 4 others at 101.4 or below. Notice that they never did it more than once in a row.

Season Sh% Save% PDO BOS 2013-14 8.5 94.0 102.5 BOS 2012-13 7.4 93.2 100.6 BOS 2011-12 9.0 92.4 101.4 BOS 2010-11 8.3 94.1 102.4 BOS 2009-10 6.7 92.6 99.3 BOS 2008-09 9.0 93.9 102.9 BOS 2007-08 7.1 93.3 100.4

In other words, a talent for PDO at the team level–a persistent ability to maintain high shooting and save rates over long periods of time–is fleeting if it exists at all. The very best record is this team beating the odds in 3 out of 7 seasons, all separated by years in which they ranged from 99 to 101.4; the Bruins’ supposed PDO talent hardly seems a bet worth making. They seem capable of being above 100 on a regular basis and above 101 about half the time, but beyond that you’re better off betting on a coin flip than on where the Bruins will land on the PDO scale.

The nature of the PDO beast is regression: as data added, the distribution narrows around the mean. Outliers are possible, but they are, by definition, exceptions and the Preds staying at 102 or above would be an outlier. An exception, not an expectation.

I’m not even sure it’s something that Preds’ fans really ought to be hankering after. It seems better to me to have a winning team whose PDO isn’t inflated than one that is. If the Preds need luck to win, then they aren’t all that good and that luck will eventually give out. Focus on the process and the luck will take care of itself.

If the Predators’ PDO remains around 102 over the rest of the season it will mean that randomness would be having an increasing influence on the Preds’ results. A PDO of 102.6 is just barely normal in December, less than half the way through the season. Over a whole season, it’s much higher, relatively speaking. In other words, unless the Predators’ PDO (especially the goaltending part of it) comes down as they play more games, they actually would be getting “lucky,” and that’s not what really where you want to be.

This assumption that 102.6 is sustainable because 6 teams have done it before is at base a prediction that the Preds will become more of a PDO outlier as data is added, which is a poor assumption to make. And you certainly can’t assume that goaltending this far above the norm for any full-season NHL goalie will continue. Put simply, Pekka Rinne’s performance to date is not predictive of where Nashville’s PDO will be in 40 more games. It’s not even predictive of where Pekka Rinne’s performance will be for the next 40 games. In other words, you can’t predict that the Preds’ PDO will stay at 102 for the rest of the season, and you especially can’t say it will happen because “Rinne is having a Hart/Vezina season.”

It’s just this kind of thinking about goaltending numbers that drives me batty for two reasons. One, goaltending is simply not predictable in 1000 shot batches. There is no correlation between any 1000 shot batch and the next 1000 shot batch. And two, a save percentage doesn’t have to be record-breaking to be good, great, or even Vezina-worthy. There are so many distortions about what’s a good save percentage and what isn’t and I spend a whole lot of my time explaining why a .920 is not bad and a .911 is not good.

More than that, none of this distortion is actually necessary. Rinne can put up a Vezina-worthy season as a whole even if his personal numbers go down. And if his personal numbers go down, the Preds’ PDO will also go down and it will still be a good season for them.

Here’s how that would work.

At this moment, Rinne has started 32 of the Predators’ 36 games, or 89%. If he were to start 89% of a full season, he would play 73 games, which is unusual but not unheard of. He’s done that once, but he averages closer to 80% of games when he’s healthy, which is 66 games. We’ll say 70 games as a compromise. That’s still more than anyone played last season but within Rinne’s capabilities if he stays healthy.

If he plays 70 games total, he’ll get 38 more games. He’s facing 22.625 shots per game at 5v5 and 28.75 shots overall. So that’s an additional 860 shots at 5v5 and an additional 1093 shots overall. His season totals would be 724+860=1584 5v5 shots and 920+1093=2013 overall shots .

Even Strength Overall Shots faced Saves Save% Season Shots faced Saves Save% Season Current 724 684 0.9448 920 859 0.9337 Above Average 860 797.22 0.927 0.9351 1093 1003.37 0.918 0.9252 League Average 860 793.78 0.923 0.9329 1093 999.00 0.914 0.9230 Below Average 860 790.34 0.919 0.9308 1093 994.63 0.910 0.9208 Terrible 860 781.74 0.909 0.9253 1093 983.70 0.900 0.9154

In other words, Rinne’s performance has been so good thus far this season, he could have a Vezina-worthy season even if he performs no better than league average from here on out. In fact, he could perform below average and still have a very good year. If he suddenly becomes terrible, he can still end the season above league averages.

I think this is why the assumption that Rinne will continue to perform at what are in truth outlandish levels kills me. Why torture logic so badly when it’s not necessary? Pekka Rinne is having a Vezina-worthy season and is likely to continue to have a Vezina-worthy season even if his numbers drop, which is far more likely than not. You don’t have to assume sustainability to achieve a great performance. You don’t even have to assume predictability to get it.

So what happens to Nashville’s season-long PDO if Rinne’s save percentage drops? Holding everything else the same and changing only Pekka Rinne’s save percentage in 70 games:

5v5 PR saves CH Saves PR Shots CH Shots Save% SH% PDO To-date levels 1496.56 177.744 1584 193.200 0.942 0.084 1.026 Above Average 1481.22 177.744 1584 193.200 0.933 0.084 1.017 League Average 1477.78 177.744 1584 193.200 0.932 0.084 1.016 Below Average 1474.34 177.744 1584 193.200 0.930 0.084 1.014 Terrible 1465.74 177.744 1584 193.200 0.925 0.084 1.009

It’s a lot more likely that the Preds will end up around 101.5 than that they’ll end up above 102. If Carter Hutton’s numbers also drop, it’ll be slightly lower. It’s a lot more likely that Pekka Rinne will end up around .935 than that he’ll stay above .940.

And none of it is anything at all to worry about.