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08-15-2012, 07:15 AM
  #57
Czech Your Math
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There's two fundamental assumptions used in GVT to value goalies.

First, it assumes that threshold level is allowing 4% more goals than league average. It's not easy to determine replacement level. I took the top 2N goalies (N = number of teams) and sorted them in terms of SV%. Median 2N is the median of the 2nd N, i.e. the SV% for goalies #45 & #46 (in terms of SV%). Median N+1 is the SV% of the 31st goalie. GVT is 4% more goals than league avg., which I hope I've calculated the same as GVT.

YEAR Med N+1 Med 2N Avg. GVT
2012 91.3% 90.4% 91.4% 91.1%
2011 91.4% 90.7% 91.3% 91.0%
2010 90.9% 90.4% 91.1% 90.7%
2009 91.0% 90.0% 90.8% 90.4%
2008 90.8% 90.0% 90.9% 90.5%
2007 90.5% 89.3% 90.5% 90.1%
2006 90.0% 89.3% 90.1% 89.7%
2004 91.0% 90.5% 91.1% 90.7%
2003 90.8% 89.7% 90.9% 90.5%
2002 90.6% 90.1% 90.8% 90.4%
2001 90.1% 89.3% 90.3% 89.9%
2000 90.5% 89.7% 90.4% 90.0%
1999 90.6% 89.8% 90.8% 90.4%
1998 90.5% 90.0% 90.6% 90.2%
1997 90.3% 89.7% 90.5% 90.1%
1996 90.0% 88.7% 89.8% 89.4%
1995 90.2% 89.0% 90.1% 89.7%
1994 89.3% 88.3% 89.5% 89.1%
1993 88.6% 87.8% 88.5% 88.0%
1992 88.6% 88.1% 88.8% 88.4%
1991 88.9% 87.8% 88.6% 88.1%
1990 88.0% 87.0% 88.1% 87.6%
1989 88.0% 87.1% 87.9% 87.4%
1988 87.8% 86.8% 88.0% 87.5%
1987 88.1% 87.4% 88.0% 87.5%
1986 87.5% 86.1% 87.4% 86.9%
1985 87.7% 86.3% 87.5% 87.0%
1984 87.1% 86.3% 87.3% 86.8%

The replacement level generally falls between median N+1 and median 2N. That seems pretty fair IMO, because teams should be able to acquire one of the better (1st-15th) backups in the league for a reasonable price. If such a backup (or sub-par starter) was actually of great value, one would expect he would at least be performing among the top 30/60 in SV% and/or he would be starting already.

The second assumption is that the difference between the goalie's SV% and league avg. is 75% the responsibility of the goalie. GVT's author mentions a 15% correlation between the SV%s of goalies on the same team, which may seem to justify such an attribution. I'm less certain how accurate this 75% attribution is, but it's not easy to support/disprove this number.

I found the data relating to PDO for the '08-'11 seasons and was recently looking at some of that. Some of what I found relates to this topic. Based on the data from BehindTheNet.com (the accuracy of which I'm uncertain at this time, e.g. shooting%s don't calculate as listed), it seems that opponents' shooting% was correlated by 40-62% from one season to the next (depending on whether using 5v5 or overall data). This may suggest that many goalies have "fluke" seasons. Perhaps the 75% responsibility needs to be substantially reduced and the random factor spread out more amongst the team (skaters), or maybe it just needs to be recognized that goalie has a lot of impact and that there's bound to be a few goalies that have big seasons each year. Besides confirming the reliability of this data (and using data from other sources), it would be helpful to see how individual goalies' opponent shooting%s (1-SV%) correlated from one season to the next.

Skaters' scoring data is compared, on a per-minute basis, to a 75% threshold. This is arbitary also. On an overall (not per-minute) basis, this seems rather high. IOW, 100 points is enough to challenge for the Ross the past couple years, but acquiring a 75 point scorer at a reasonable price seems much less likely than finding a goalie who is not getting full playing time. I tend to believe that this could be an important cause of the disparity between top goalies and top forwards in GVT.


Last edited by Czech Your Math: 08-15-2012 at 09:44 AM.
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