2007-08 NHL Goal Differentials
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06-12-2008, 12:12 PM
Join Date: Oct 2005
Originally Posted by
Dr. No, could you explain how exactly you determine a goalies record with "average goal support"?
I apologize - I missed this post back when it was posted, and only noticed it when I linked to it just now.
Here are the basics - first, I ignore ties. That makes things a hell of a lot simpler and doesn't really seem to bother anyone.
I'll introduce Bill James' Pythagorean Theorem - in baseball, if you know how many runs scored and runs allowed a team has in a season, you can effectively predict their record using the formula
Winning Percentage = Runs^2 / (Runs^2 + RunsAllowed^2)
. The formula works pretty well for hockey - it's not perfect, but then again, we're starting from save percentage, and that's already not perfect, so there we are.
So the hockey formula becomes
Winning Percentage = GF^2 / (GF^2 + GA^2)
Goals Against is easy to find - that's just the Goals Against that a goaltender allows.
In a support-neutral context, Goals For is basically asking - if this goaltender faced "league average" goaltending in similar situations, how many goals would his team score? The simple answer is that you take the number of shots the goaltender faced and multiply it by (one minus the league-average save percentage).
Once you have GF and GA, you can get Support-Neutral Winning Percentage using the formula above.
Then you can take the SNW% and apply it to how many decisions the goaltender had over the season.
Here's an example, using Martin Brodeur's 2007-08 campaign. Brodeur had a record of 44-27-6, allowing 168 goals on 2089 shots. League-wide save percentage was 0.90881 (I remove Brodeur's shots and saves in making this calculation because Brodeur doesn't play against himself, and he shouldn't get penalized for playing well).
Goals Against = 168.
Goals For = (1-0.90881) * 2089 = 190.5
(Note that I would ordinarily carry as many decimal places as I could, and then round at the end)
SNW% = (190.5^2) / (168^2 + 190.5^2) = 0.5625.
Brodeur had 77 decisions, so his support-neutral wins would be (0.5625 * 77) = 43.3 (rounded to 43). And his support-neutral losses would be 77-43=34.
So Brodeur's 2007-08 support-neutral won-loss record would be 43-34.
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