I thought I would take a quick look at the effect of defensemen on save percentage, relative to their teammates. This is just for 5v5 in the NHL during the past two seasons.
To do this I used Desjardins' terrific behindthenet statistics site
. I arbitrarily set the cutoff at 30 games played, and grabbed the 08/09
From there I took the on-ice save percentage and subtracted the off-ice save percentage. The off-ice save percentage applies only to the games in which the player was on the roster. If you're doing this yourself, note that the shots on Gabe's site are actually saved shots, you'll need to add goals against to get total shots. Also empty net goals are included in the data, which is unfortunate, and is going to make the offensive defensemen on bad teams seem a touch worse.
By way of example, in 08/09 Chris Pronger had a 5v5 on-ice save percentage of .915
. When he was in the game, but not on the ice, the opponents scored at a 2.20 goals per 60 clip, and the ducks made saves at a 26.5 saves per 60 clip. So 26.5/(26.5+2.20) = .923. So, in 08/09 Chris Pronger had a 5v5 off-ice save percentage of .923
. His net score was -.008, meaning that the Ducks goalies stopped pucks at a .008 better clip when he was on the bench at 5v5, rather than on the ice. We'll call that his 5v5 save percentage score
This season Pronger was +.008.
The same exercise was repeated for all players.Real Effects:
If there is a real ability of defensemen to affect shot quality, it should repeat from season to season in the general population. So below is a scatter plot of 08/09 vs 09/10 save percentage scores for all defensemen who played at least 30 games in each season. Click to enlarge.
The guy at the top right is Mark Fistric, the player to the far middle right is Brett Festerling, and Jack Johnson is the player the furthest to the bottom left. These aren't outliers in the true sense of the word, the universe requires that some guys have this level of good and bad fortune. In fact if you remove them, the correlation from season to season becomes negative, and the bunching of the dots becomes far more tightly grouped than we would expect by random chance.
In fact, the bunching is tighter than we would expect by random chance already. The reason is NOT survival bias (a phenomenon that corrupts many MLB stats beyond recognition) coaches and GMs in this league seem to have this stuff figured out. You could make a case that Alzner would have made the Caps this year if he'd been luckier in 08/09, and that Hedican probably had another year or two left in him. But that's just two guys and both are at the edges of their careers, it's a non-issue on the whole.
There is some censorship bias though. The group is bunched a bit too closely together because good defenders tend to play more against good forwards, so their 5v5 save percentage scor
e suffers a smidgen. And if a guy is playing tough minutes and getting shelled in terms of save percentage score, coaches tend to give them a break from the tough gig, probably just to make sure that they don't start losing confidence. So the correlation of Corsi QualComp to save percentage score is only r=-.01. But this defender was playing tough minutes in the first place because he's a good player, the coaches know that, so next year he'll be back to facing tougher comp. So the correlation of 08/09 Corsi QualComp to 09/10 5v5 save percentage scor
e is a touch stronger, r=-.09.
In terms of old school QualCOMP, which is a tad self referring. postdictive r=-.05, predictive r=-.10
The same phenomenon occurs from 07/08 to 08/09, though not quite as strongly.Bottom Line:
The ability of defensemen to affect shot quality against does exist in the population, but it is so small that we will never be able to sensibly apply it to any player in particular. And a paradox is created, the type of defensemen who are helping the goalie save percentage a bit (presumably because they make fewer mistakes of the spectacularly bad variety) are, as a group, seeing slightly worse
save percentages behind them, because they are the guys the coaches are leaning on to play tougher opposition. And the guys who have talent but are guilty of the occasional egregious error ... as a group, they do a whisker better than average by 5v5 save percentage scor
e. This is presumably because their coaches have the good sense not to play them much against Malkin, Kovalchuk and Heatley types.Trailing Thoughts:
Even though it will not be popular with fans, I think the right guys for a team to target are defenders who have been getting some bad bounces in recent seasons. They should come cheaper in trade than their true value dictates. They are:
1. YouTube clowns. Souray was the poster child for the phenomenon. Plus he had a brutal save percentage score his last year in Montreal, but had a decent level of competition (Carbonneau wasn't fooled) and decent Fenwick numbers (and therefore almost certainly decent on-ice scoring chance numbers). That save% score bounced back in a ridiculous way for his first two years as an Oiler, and plummeted this season, creating the illusion that Charlie Huddy was a supergenius. The dice have no memory, after all.
2. Guys with back to back seasons of poor 5v5 save percentage scor
es, three in a row is even better. NHL teams aren't swayed much at all by these (oddly enough, it is the implicit core focus of fandom) but no matter how square your head is ... two or three seasons in a row of bad luck and the mind searches for reasons. Just watch a craps table for a couple of hours if you don't believe me, and that's with dice. So it's bound to affect at least some GMs. The universe requires it happens to some defenders, so I think these guys could come cheaper than their true market value. Zbynek Michalek fits the bill, Colin White, Bieksa and Jack Johnson maybe. Niedermayer isn't going anywhere, unfortunately, but he's been rolling snake-eyes over the last couple, creating the illusion of a sharp decline in his play.
3. Scoring chance % may not be everything for defenders, but it's almost everything. And it repeats really well just as a raw number. If you apply LupulSmid
to account for quality of D partner, then the season to season correlation goes through the roof, r=.85 for the Oilers D, and that's without accounting for the rest of the contextual issues (quality of competition, how often they start in their own end of the rink, etc.). So it's time to move a guy like Smid, before the bottom falls out of his save% score. Because it will eventually.