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07-20-2013, 11:25 PM
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Originally Posted by Brainiac View Post
I'm not sure I agree completely with this definition of PDO. I know it's the official definition and all but still.

The average PDO (weighted for #shots) for the whole league will be exactly 1000, that's obvious. But a good team could be consistently over 1000 and a bad team could be consistently under. By definition, it doesn't have to regress to 1000 for one specific team. Or it's not entirely due to luck.

A very skilled team can finish a season with a PDO of 1020 or 1030. It's not luck anymore, IMO. Take an NHL team and send it to the AHL and I would bet my house that its PDO won't 'regress towards 1000'.

What could be considered 'lucky' is to have a very high PDO (like 1050 or 1100) over a stretch because of hot goaltending or unusually high shooting%. But that's it.
Here's the thing with PDO: It's a great concept and the idea behind it makes a ton of sense: Hockey is a game played on a not-entirely-smooth ice surface with ten guys skating around at high speed. Where exactly a bouncing disk of rubber goes is sometimes decided by luck. A bad bounce, etc.

The problem with PDO is that it generally regresses towards a mean of 1000 (on-ice save percentage of ~920 (.920%) + on-ice shooting percentage of ~80 (8.0%) at even strength, but not really. Yes, that's a contradiction, but let me explain my thinking.

There have been a lot of recent studies that suggest two interesting things: Defensemen have a tiny bit of control over their on-ice save percentage and none over their on-ice shooting percentage. Likewise, forwards have a tiny bit of control over their on-ice shooting percentage but none over their on-ice save percentage. Likewise a very poor defensive defenseman would allow a greater amount of quality of shots on his goaltender and therefore earn a below-average on-ice shooting percentage.

So I believe that a defenseman who is very good defensively at blocking/diverting/directing elsewhere/keeping elsewhere shots can have a little effect on the on-ice save percentage of their goaltender. Likewise, I think that a very creative, talented offensive forward can influence to a small degree the quality (location/speed/type/etc) of shot they direct at the net. 4th line face-punchers, on the other hand, rarely take shots that are not from 30 feet out with no screen.

However, I think the range of these things is very small (see #1 at bottom of post). Example: Here are Sidney Crosby's on-ice shooting percentages from as long as the stat has been measured. See how large the range is? But here's another subtle thing to notice; as the sample size gets larger, the on-ice shooting percentage gets lower. The season he played just 22 games, he had a huge 15.17% shooting. The two seasons he played close to the full 82 games (in this sample, which is just the seasons in which BTN tracked stats) he had a 11.02 and 11.54 on-ice shooting. I think if the rest of his injury-riddled seasons had been completed, you would have seen his on-ice shooting regress to a more reasonable 11%. And Crosby is the very top of the food chain. I think he sets the upper limit. Now, just browsing through BTN, by eye I would guess the lower limit for the 5 minute per game 4th line facepunchers is about 6%, with more players at this end than at the Crosby end (there are endlessly more 4th line facepunchers than Crosby's in the league). I think that if you ignore the facepunching 4th line grinders and the elite talents like Crosby* (the former because who cares about the offensive talent of 4th line facepunchers and the latter because players like Crosby are outliers in the NHL), you'd get a true talent range of between about 7.5% and 9.0% for forwards (and I feel I may actually be being generous here. See #2 for a good read about the sustainability of on-ice shooting percentages). Now considering that there is a good amount of data suggesting that forwards have little to no effect on their own on-ice save percentage, that puts true talent PDOs for individual non-facepuncher non-Crosby NHL forwards at a range of 995 and 1015, assuming league average goaltender. That's a very small range. And that's why PDO is so safe, because even if you're accounting for differences in true talent on-ice shooting percentage, the rest is so very uncontrollable for that forward and is thereby susceptible to fluctuations in luck.

Now compare the range of likely true talent to the range of on-ice shooting percentages you see in individual seasons. That's where the luck plays in.

If I were so inclined, I would look at the range of true talent on-ice save percentages for defenseman, but for simplicity's sake, I'll just say it's .10% at the extremely of defenseman's capabilities. I think that's generous, I think a truer estimate would be closer to .05%. Of course, the thing about on-ice save percentage is that it is largely based on the true talent of that skater's team's goaltender. But on average, the league's goaltender's put up a .920, so on average the true talent PDO of a defenseman would be 995-1005. Again, this is a much smaller range than what we often see on a season-by-season range (see #3 for a good read on the sustainability of defensemen's on-ice save percentages). Plus, if you look at the on-ice save percentages of defensemen in any given year, the names look positively random (see #4). So I think it's a safe assumption that forwards can affect shot quality for more than defensemen can affect shot quality against.

But regardless, here's what you should take away from this:

1) In a basic investigation, assuming a true talent PDO of 1000 is safe, no one should freak out about it. Anyone who cries about a 1010 PDO being hugely inflated clearly doesn't know what they're talking about and probably shouldn't be trusted in this sort of analysis.

2) a) When evaluating a forward on an individual basis, look for a true talent PDO comprised of the career even strength save percentage of the team's goaltender plus the forward's career average on-ice shooting percentage, and then look for season-to-season deviations to determine how lucky/unlucky that player's season/stretch of games/whatever sample size you're looking at has been.

b) When evaluating a defenseman on an individual basis, look for a true talent PDO comprised of the league average shooting percentage (8%, which includes both the elite talents like Crosby and 4th line facepunchers, as defensemen face all players, although I'd love to see eventually a stat which averages the career on-ice shooting percentages of players a defenseman faces weighted by ice-time against, but that would be a huge project) plus the career average of the defenseman's goaltender the established effect that defenseman has had on his goaltenders over his career, and compare that to the season/sample size you're looking for the luck factor over.

3) The funny thing about PDO is that very often a high on-ice shooting percentage is coupled with a low on-ice save percentage, resulting in a PDO of, wait for it, 1000. Why is that? Is it because high-octane offensive forwards with true talent on-ice shooting percentages are likely to be paired with great offensive but poor defensive defensemen who cause a true talent deflation of their goaltender's save percentage behind them? Or is it a system-based deal, where run-and-gun systems cause higher quality scoring chances at both ends and conservative defense-first systems cause lower quality shots at both ends of the ice? I would bet more on the former, but it'll take someone more versed in systems and analytics than I to determine that.

4) LUCK IS AN ACTUAL PHENOMENON. There is a degree of shot quality on an individual basis but it is not as large as what people who scream about shot quality would have you think. A PDO of 970 or 1030 (see #5) (like the poster I'm quoting alludes to) WOULD be in fact very lucky, even if that team had very good goaltending and a high proportion of high true-talent shooters.

So while I see the flaws in PDO and the ways it could be improved, I still think it's a very important concept that should not be disregarded.

*Crosby is one example, other forwards that appear to be outside the general range are Ovechkin, Stamkos, Malkin, Semin, Getzlaf.






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