There was an interesting discussion that kicked off on Twitter about defencemen and ZoneStarts and this is a sort of thing that hockey analytics is going to need to get into eventually. As it so happens, I’ve been digging into this recently, and I have a bunch of data handy that will shed light on a few things.
There are 1269 defencemen who were on the ice for at least 100 faceoffs in a season between 2007-12. (I’m counting seasons differently, so an individual defenceman can appear as many as five different times.) We’ll start with ZoneStart and Corsi.
At the risk of pointing out the obvious, you can see that the Corsi% numbers are clustered much more tightly than the ZoneStart numbers. There are 219 NHL D-seasons between 2007-12 with a ZoneStart at or below 44.9. There are only 149 NHL seasons with a Corsi% at or below 44.9%. The flip side of that is that there are 267 NHL D-seasons at or above a ZoneStart of 55.0 and only 100 seasons with a Corsi% above 55%. Corsi% for defencemen is, for all intents and purposes, clustered between 40% and 60%. ZoneStarts aren’t.
It’s interesting to me that defencemen seem far more likely to put up low Corsi seasons than high Corsi seasons. I suspect that there’s a simple truth about how hockey works that explains this. Imagine that you’re Marc-Andre Bergeron, a ZoneStart All-Star a few times. Despite getting all of those starts in the offensive zone, you’re still going to spend a significant chunk of time in your own end because defencemen only change going one way – towards the offensive zone. Forwards can get away with changing while the puck is heading the wrong way. Even if you start in the offensive zone, unless your team keeps the puck there for an entire shift, you aren’t getting off the ice without the puck coming out and likely coming into your own end.
It’s different for guys with low ZS. Nick Schultz is the all-time champion here, posting a ZS of 26.5 for the 2008-09 Minnesota Wild. In Schultz’s case, there was no guarantee he’d be on the ice for a slice of offensive zone time that corresponds to the guarantee that guys who have high ZS will be on the ice for a slice of defensive zone ice time. Whenever the puck leaves the defensive zone, Schultz can probably change.
The ratio of D-seasons with low ZS to D-seasons with low Corsi% is 149/219 or 68%. For high ZS to high Corsi%, it’s 100/267, or 37.5%. The way in which defencemen change in hockey games would provide an explanation for that, I think.
Local astronomer Bruce McCurdy asks whether we don’t lose something by discarding neutral zone draws from ZoneStart. As readers are presumably aware, the ZoneStart formula is (OZ faceoffs/OZ faceoffs + DZ faceoffs). Bruce quite reasonably points out that “…there’s a difference between,say 30D +50N + 20O & 60+0+40, even as both are 40% OZ%.” The assumptions inherent in ZS is that everyone’s going to get about the same number of neutral zone draws and that there aren’t faceoff effects as a result of them. I did a quick check of my sample of defencemen to see whether that holds.
Truthfully, I think it holds up pretty well. 1156 of the 1269 D-seasons under examination saw between 35% and 45% of the faceoffs that they were on-ice for take place in the neutral zone. I doubt that it’s a significant factor, except possibly at the absolute extremes, although I would expect it to show up more in the rate at which Corsi events take place when a player is on the ice than in the percentage of them that his team achieves.
Colby Cosh comments that the effects of ZoneStart on Corsi% seem to be non-linear. I think that there’s something to this. I’ve graphed the average Corsi% for the six groups of players with ZoneStarts between 35-39.9 and 60-64.9. As ZoneStart increases, Corsi% increases. There’s something going on though – for the 35-39.9 group, the average Corsi% is 44.7%. For the 60-64.9 group it’s 53.3%. Both groups are 10-14.9 percentage points from 50% and yet the decrease sustained by the group with tough ZoneStarts is 5.3 percentage points from the mean and the increase achieved by the group with easier ZoneStarts is just 3.3 percentage points north of 50%. I’d guess that this is tied into what I mentioned before, about how and when defencemen change.
I’ll throw out one other graph here. I went through and created pairs of D-Seasons to examine. Any defenceman who was on the ice for at least 100 faceoffs in consecutive seasons constitutes a pair. So, 2007-08 Zbynek Michalek, who had a 51.1% Corsi% with a 55.3 ZS is paired with 2008-09 Michalek, who had a 43.4% Corsi% with a 35.5 ZS. For each pair, I then took the difference between the Corsi% in the two seasons and the ZS in the two seasons. There’s actually a reasonably strong correlation coefficient between those two numbers – 0.5. Accordingly, as a player’s ZS increases or decreases, his Corsi% tends to move along with it.
What can we take from this? I’m a believer that ZS matters, although I suspect that it hurts guys with a tough ZS more than it helps guys with an easier one. I doubt that the neutral ZoneStarts skew things all that much, although it’s probably worth checking whether or not guys with higher shares of NZ ZoneStarts tend to do differently than guys with lower shares of NZ ZoneStarts. That’s kind of where I think we are on this.Email Tyler Dellow at email@example.com