October 28 2011 09:22AM
I've been scratching my head for a while now, trying to come up with a puck possession metric that focuses on defense rather than offense-driven numbers. I mean, if you're Anton Volchenkov, you might be responsible for preventing a lot of goals, but there's always the chance that last year's New Jersey forwards were incapable of putting shots on the opposition's net, and that your Corsi numbers look bad. What's a defensive defenseman to do? There's got to be a way of measuring defensive contributions that isn't tied to shots or goals.
So here's my rationale: only the puck carrier can be hit, so if there is a lot of hitting in your end of the ice, the puck must be there a lot, right? Even if your team has the puck, and is getting hit a lot in their zone, doesn't that mean that you're struggling to get out? Leafs fans should certainly remember last year's broken-record recording of "to the line, but not out", so anecdotally, anyway, this makes sense.
I've done some work similar to this before, where I calculated whether teams as a whole were hitting more often in the offensive zone or defensive zone, but only recently, with the help of the wonderful Eric T (follow him @BSH_EricT) from Broad St. Hockey, was I able to get individual numbers for players.
What we did was to tally every single hit that each player was on the ice for - that is, not just when each player was being hit or was hitting (that would only reward players that hit a lot) - and determine which zone it came in. Theoretically, if players' teams hit more often in the defensive zone when a given player was on the ice, they're working more frequenty to recover the puck there.
To see whether or not this makes sense as a puck possession metric on an individual player level, I've graphed my new statistic called Zone Hit Ratio (ZHR) to see if there is any correlation to Corsi numbers. Now, I never wanted too close a correlation, because we're trying to measure something different than Corsi is, but the results were nevertheless underwhelming. This graph shows each player's ZHR compared to their Corsi in 2010-11:
So, looking at our ZHR axis, anything over 0.5 means that more than half of the hits a player was on the ice for were in his own end. There's actually a very small correlation between having a good Corsi and hitting more frequently in your own end. Of course, it's such a slight connection that it's more than likely just statistical noise. Given that p=0.174 in this case, I don't think there's much to write home about.
On the other hand, 'who is doing the hitting' has a stronger correlation with Corsi than where the hitting is going on. Here, we're looking for a relationship between a player's Hitter/Hittee ratio (again, for all players while he was on the ice). In other words, "was your team out-hit, or did your team hot more than the opposition while you were on the ice"? All data from the 2010-11 season, once again.
Here, we have a significantly stronger correlation, with a "p" value of 0.021. It's not the greatest thing I've ever seen, but it's worth talking about. It appears that if your team is controlling the play, they're going to be the recipients of more hits. Makes a certain amount of sense, right?
The only downfall of this is that it's not zone-specific, so we can't get at who is doing the most work in the defensive end, but it is a way of looking at who controls the play without examining shots or goals.
I confess to being somewhat surprised at the lack of connection between Corsi numbers and ZHR, which should measure attempts to regain puck possession, but then, there are a number of points to be considered, here. Maybe that there isn't any connection isn't so perplexing, after all.
First, is the unreliability of the data. If we can't even count on the NHL to get faceoff locations right, deciding what is/isn't a hit would be a nightmare of subjectivity.
Second, hitting an opposing player who has the puck is only one way of regaining puck possession. In fact, a well-timed stick lift or poke check is often a better option, because these other options don't take you out of the play. So measuring hits isn't anywhere close to measuring every single puck contestation that occurs in the game, and thus paints an incomplete picture of where defensive work is being done. Of course, puck contestations are far from the only way that puck possession changes, so this also neglects turnovers, dump-ins, and the like. My hope with this study was that hits happened frequently enough (and thus, with a big enough sample size) to measure puck possession in each end of the rink. Not so.
That there is a stronger relationship between the Hitter/Hittee ratio and Corsi is interesting, and merits further investigation.
I'd love to hear your thoughts/ideas on how to go about developing this, or other related ideas, so drop me a line on Twitter (@JPNikota) or leave your comments here.