Zone Start Adjusted Corsi
The idea is simple: take a player's ice time and use the league average Corsi for each type of start to determine what an average player's Corsi would be with the same ice time. Subtracting that off will give you how much he is above, or below, what the average player would get with his ice time. To see how this works, let's look at the poster child for zone-start adjustment, Manny Malhotra. Here is a chart summarizing Malhotra's time in each start, along with his Corsi numbers:
|Manny Malhotra||Time (mins)||Corsi / 60|
|Ozone, first shift||55.2||48.913|
|Ozone, after change||170.8||-7.376|
|Dzone, after change||142.3||-13.073|
|Dzone, first shift||178.7||-31.234|
Here are the league averages for each type of ice time:
|League Average||Corsi / 60|
|Ozone, first shift||40.147|
|Ozone, after change||2.818|
|Dzone, after change||-2.818|
|Dzone, first shift||-40.147|
Weighting by Malhotra's ice time gives us -5.496, meaning that if someone performing at the league-average level was given Mr. Malhotra's ice time he would have a Corsi of -5.496. To get Manny's Zone Start Adjusted Corsi we subtract that off, in other words add 5.496, to get -3.769.
A Rule of Thumb: Simplified Zone Start Adjusted Corsi
That's all well and good, but it would be nice to have something a little more portable. Even with all the data, I'd like to be able to just pull up BTN and get an idea how to adjust for a guy's Ozone%. To get something simpler, I recorded the Ozone% according to BTN for all of the players with at least 600 minutes of even-strength-goalies-on ice time and ran a regression to get the average adjustment for a given ozone%. Here is a scatter plot of the 508 players. The numbers on the x-axis represent how far off from 50% Ozone%, the y-axis is the size of the adjustment or the negative of what the average player would get with the same ice time:
As you can see, a simplified formula will come very close to the more complicated version above which forces us to look at the individual data. Any differences are based on how much time a player spends in the relatively neutral situations where he is jumping on the ice after a faceoff at either end. The result of this is a simple formula. To adjust for zone starts, multiply how many percentage points the player's Ozone% is from 50% by 0.18 and add or subtract accordingly. In formula, with Ozone% out of 100:
Simplified Zone Start Adjusted Corsi = Corsi/60 - (Ozone% - 50)*0.18
Another way to think about it is to add or subtract 1.8 for every 10 percentage points. So if you gave a guy with even zone starts 60% Ozone starts then we'd expect his Corsi rate to go up 1.8. If you put him in more defensive spots with just a 30% Ozone% then his Corsi will drop about 3.6.
I don't want to clutter it with a 900-row table, so I'll make a table with the top 25 and another with a few players of interest with particularly high or low Ozone%. Here is a google spreadsheet with all the Zone Start Adjusted Corsi stats from 2010-2011.
|Rank||Player||Team||Pos||Zone Start Adjusted Corsi||Corsi||Time On Ice|
|2||Torrey Mitchell||SJS||F||18.504||18.336||791.9||3||Joe Pavelski||SJS||F||17.304||15.939||1039|
People of interest:
|Player||Team||Pos||Zone Start Adjusted Corsi||Corsi||Time On Ice|
Please Leave Feedback!
As this is my first effort in coming up with a new statistic, I would love some feedback on this. Does the methodology make sense? Is the Ozone% adjustment of .18 per percentage point pretty close to what you've been doing? Any and all comments appreciated.