Tuesday, April 24, 2012

NHL Goalie Performance Measure

Here is a step-by-step guide to David Berri's and my NHL goalie measure from our article in the Journal of Sports Economics in 2010.

Step 1: Calculate the Marginal Value of a Goal Against. I have previously posted on how to do this, so follow the steps in this blog.
Step 2: Download NHL Goalie Data I downloaded the 2011-2012 NHL goalie data from NHL.com. Go to the web page, click on Stats, then choose Individuals. Once that page opens, I choose 2011-2012 Regular Season and under Position I choose Goalie. This opens up a page with all the NHL goalies for the time period you are analyzing. Make sure you get all the goalie data by choosing the additional tables at the bottom of the first table. I then copied and pasted each table into Microsoft Excel.
Step 3: Wins Above Average (WAA) Calculation. WAA is measured as the absolute value (since a goal against has a negative effect on team wins) of the marginal value of a goal against divided by two (since each win is worth two standings points) times the number of shots on goal that goalie faces times the difference in the save percentage of the goalie and the average save percentage of all goalies for that season.  Let's break this calculation up into three parts.
Part A:  This is already done, since the data is just the columns listed as SA and SV% from NHL.com for each goalie, which in my spreadsheet is column I for SA and column M for SV%. 
Part B:  Calculate the average save percentage for the league by finding the total number of shots against and the total number of goals against and then subtract the total number of goals against from the total number of shots against and divide that by the total number of shots against.  In Excel this would be (S6-S9)/S6, if total shots against is in cell S6 and total goals against is in cell S9.  Suppose this formula is put in cell S12.
Part C:  The first part of the equation [the marginal value of a goal against] was found in step 1.  Take the absolute value of this number and divide by two.  If the marginal value of a goal against is in cell S3, then in Excel type ABS($S$3)/2.
Put it all together and in Excel we have the following formula:  =(M2-$S$12)*I2*(ABS($S$3)/2).  Note that M2 is the first goalie listed save percentage and cell I2 is the first goalie listed shots against.  Then copy and paste this formula down in the spreadsheet for each goalie and you have an estimate of each goalie's Wins Above Average (WAA) for that season.
Step 4Rank Most Productive Goalies.  I then rank each goalie by their WAA from highest to lowest for that season.  You can do this by sorting from highest to lowest.  I do something slightly different but it is the same thing, it just allows me to do this without actually having to use the sort option in Excel.

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