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# Calculating GVT

Turns out it's harder than you think.

In a previous post, I looked at simulating GVA. Up next, I plan to look at GVT. First, though, I needed to see if I can accurately calculate GVT. Tom Awad outlines how to calculate GVT in his "Understanding GVT" series. In Part 3, http://www.puckprospectus.com/article.php?articleid=236, he explains how to calculate goaltender GVT (GGVT).

Threshold save percentage (Spt) is defined in terms of average save percentage. There is a typo in the equation in the article at this point. Let me define Yield (Y) as Y = 1 – SP. Threshold yield Yt is defined as 4% more than average yield, or Yt = Ya * 1.04. Thus, SPt = SPa - GTV * (1 - SPa)

 Save Percentage ES PK PP Total Average 0.917 0.868 0.914 0.909 Threshold 0.914 0.863 0.911 0.904

Expected goals are calculated as shots faced times save percentage. Expected goals minus actual goals gives GGVTraw. This is then multiplied by 0.75 to get GGVT. Two additional components, for puck-handling and shootout are added to GGVT to get total GVT. I'm not going to calculate these, but they are small relative to GGVT. Seems easy enough, but I'm not getting the same values Gabe has in his tables. I calculated a piece of GGVTraw at ES, another on PK, and a third on PP. These are added up. For example, for Ryan Miller, I get expected goals against as 197.06, GGVTraw of 47.06, and GGVT as 35.3. Gabe's tables have it as 38.5. Similarly, for Vesa Toskala I get GGVTraw as -18.75 and GGVT as -14.1. Gabe's tables have him at -13.8. In fact, my calculated GGVT values average 1.12 less than Gabe's tables. The appendix has the calculated GGVT values for 2009.

I then calculated GGVT based on total shots and total save percentage. Miller 38.56, Toskala -14.1. Still not quite right. These values are low by 0.51 on average.  The appendix also has these values.

I'm going to declare victory and move on. I'm going to calculate GGVT a piece at a time, ES plus PK plus PP. I'll call what I calculate GGVT* to make it clear that what I am calculating is not quite GGVT, although the difference is small, much smaller than the standard deviation we saw with GVA. I suspect the difference in GGVT and GGVT* reflects a difference in calculating shot mix.

Acknowledgment

Appendix

GVTcalc : the GVT calculated from ES + PK + PP

GVTact : GVT from Gabe's tables

Diff1 : difference between GVTcalc and GVTact

GVTtot: the GVT calculated from total shots

Diff2 : the difference between GVTtot and GVTact

 Name GVTcalc GVTact Diff1 GVTtot Diff2 MillerRyan 35.30 38.50 -3.20 38.56 0.06 VokounTomas 29.29 31.50 -2.21 32.08 0.58 NabokovEvgeni 28.28 30.50 -2.22 28.60 -1.90 HowardJimmy 24.95 26.80 -1.85 27.38 0.58 BryzgalovIlya 24.06 26.00 -1.94 24.91 -1.09 KiprusoffMiikka 23.97 25.70 -1.73 24.19 -1.51 LundqvistHenrik 23.66 28.30 -4.64 24.27 -4.03 RaskTuukka 23.23 24.90 -1.67 26.60 1.70 AndersonCraig 20.15 21.90 -1.75 21.04 -0.86 HalakJaroslav 19.26 21.70 -2.44 21.28 -0.42 HillerJonas 18.53 20.90 -2.37 19.92 -0.98 BrodeurMartin 13.54 17.10 -3.56 18.29 1.19 WardCam 12.20 13.10 -0.90 12.20 -0.90 LuongoRoberto 11.60 14.50 -2.90 10.01 -4.49 MasonChris 11.53 13.80 -2.27 11.31 -2.49 HedbergJohan 11.21 12.30 -1.09 8.30 -4.00 ConklinTy 10.40 10.90 -0.50 11.33 0.43 TurcoMarty 10.00 10.60 -0.60 12.63 2.03 LaBarberaJason 8.41 8.70 -0.29 10.56 1.86 ThomasTim 8.37 9.60 -1.23 9.91 0.31 PriceCarey 6.87 8.40 -1.53 4.01 -4.39 TheodoreJose 6.41 7.40 -0.99 7.82 0.42 NiittymakiAntero 6.08 6.10 -0.02 3.77 -2.33 RinnePekka 5.56 7.70 -2.14 5.89 -1.81 NiemiAntti 5.38 6.00 -0.62 6.59 0.59 ElliottBrian 5.22 6.60 -1.38 2.71 -3.89 BrodeurMike 4.13 4.10 0.03 4.69 0.59 BernierJonathan 3.64 3.80 -0.16 3.79 -0.01 BudajPeter 3.02 3.90 -0.88 2.90 -1.00 KhudobinAnton 2.81 2.70 0.11 8.20 5.50 ClemmensenScott 2.73 3.70 -0.97 5.03 1.33 NeuvirthMichal 2.72 3.60 -0.88 3.41 -0.19 QuickJonathan 2.69 5.50 -2.81 3.85 -1.65 DanisYann 2.44 2.80 -0.36 2.48 -0.32 GiguereJean-Sebastien 2.36 2.80 -0.44 1.95 -0.85 EllisDan 2.11 2.90 -0.79 1.47 -1.43 GreissThomas 1.99 2.80 -0.81 2.29 -0.51 RaycroftAndrew 1.98 2.90 -0.92 0.50 -2.40 PavelecOndrej 1.85 2.50 -0.65 0.19 -2.31 VarlamovSemyon 1.84 3.70 -1.86 0.27 -3.43 LehtonenKari 1.83 1.90 -0.07 0.23 -1.67 KhabibulinNikolai 1.58 2.20 -0.62 1.10 -1.10 RolosonDwayne 1.49 3.40 -1.91 2.09 -1.31 FleuryMarc-Andre 1.41 3.90 -2.49 2.95 -0.95 JohnsonChad 1.24 1.60 -0.36 3.31 1.71 LalimePatrick 1.20 1.10 0.10 0.35 -0.75 LegaceManny 0.84 2.50 -1.66 2.23 -0.27 EmeryRay 0.61 2.10 -1.49 -0.35 -2.45 ErsbergErik 0.60 0.60 0.00 -0.46 -1.06 LeightonMichael 0.40 2.00 -1.60 -0.34 -2.34 SchneiderCory 0.33 0.50 -0.17 0.10 -0.40 JohnsonBrent 0.24 1.30 -1.06 1.89 0.59 CrawfordCorey 0.20 0.20 0.00 0.70 0.50 PechurskiAlexander 0.20 0.20 0.00 -1.10 -1.30 BacklundJohan 0.17 0.20 -0.03 -1.30 -1.50 McElhinneyCurtis -0.13 2.90 -3.03 -1.01 -3.91 ZabaMatt -0.24 -0.30 0.06 0.88 1.18 PetersJustin -0.31 0.30 -0.61 -0.59 -0.89 EnrothJhonas -0.41 -0.40 -0.01 -1.62 -1.22 DuchesneJeremy -0.45 -0.40 -0.05 2.17 2.57 GaronMathieu -0.54 0.60 -1.14 -0.53 -1.13 HardingJosh -0.56 0.50 -1.06 -1.45 -1.95 TokarskiDustin -1.18 -1.10 -0.08 0.98 2.08 DubielewiczWade -1.21 -1.20 -0.01 3.74 4.94 ClimieMatt -1.22 -1.00 -0.22 -2.06 -1.06 DipietroRick -1.30 -0.30 -1.00 0.32 0.62 SalakAlexander -1.41 -1.60 0.19 3.21 4.81 MacDonaldJoey -1.42 -1.40 -0.02 -0.47 0.93 CurryJohn -2.16 -2.70 0.54 1.58 4.28 GustavssonJonas -2.56 -1.20 -1.36 -5.03 -3.83 BackstromNiklas -3.26 0.40 -3.66 -1.49 -1.89 BoucherBrian -3.69 -1.40 -2.29 -2.69 -1.29 MasonSteve -3.99 -1.50 -2.49 -4.90 -3.40 BironMartin -4.40 -4.70 0.30 -4.08 0.62 ValiquetteSteve -4.96 -4.80 -0.16 -1.00 3.80 SmithMike -5.07 -2.60 -2.47 -6.31 -3.71 AuldAlex -5.24 -3.80 -1.44 -3.87 -0.07 DeslauriersJeff -5.32 -3.60 -1.72 -3.23 0.37 DubnykDevan -6.31 -6.20 -0.11 -3.91 2.29 OsgoodChris -7.86 -6.80 -1.06 -6.86 -0.06 HuetCristobal -8.23 -7.00 -1.23 -7.52 -0.52 LeclairePascal -9.77 -9.70 -0.07 -10.57 -0.87 ToskalaVesa -14.06 -13.80 -0.26 -14.09 -0.29 Average Difference -1.12 -0.51