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# Comparing GGVT to GVA

For goaltenders, GGVT is the same as GVA.

I've been looking at simulating goaltender performance and calculating GVA and GVT. Here, I look at actual GGVT compared to actual GVA and the simulation data for GVA. For example, in 2009 Roberto Luongo had a observed GGVT of 11.1 and an observed GVA of 9.03. Simulating his workload, I found that GVA had standard deviation of 12.58 at that workload. Thus a 95% confidence interval for his GVA is [33.68, -15.62]. Since Luongo's GGVT falls in this interval, it is statistically the same as his GVA.

I compared actual GGVT to the calculated confidence intervals for GVA for all 434 goaltenders who played at least 1 game from 2005 to present. The 2009-2010 data is presented in full in Appendix A below. The data for all seasons is here http://spreadsheets.google.com/ccc?key=0An-GS2MUpN1mdFJsZE1veTU4a1o3cjRwTlEtMW43a3c&hl=en&authkey=CP7-uq8F Every single one of the 434 actual GGVT values observed fell within the corresponding 95% confidence interval for GVA.

In most cases, the differences between actual GGVT and GVA are small. There are 30 goalies for whom the difference is greater than 10. 28 of them are from 2005 and 2006. The other 2 are from 2008. For example, Martin Gerber in 2005 has the greatest difference. His actual GGVT was 11.8 but his GVA was -10.4 for a difference of 22.2. I calculate a GGVT* of -3.64, which is a lot closer to his GVA. I have listed the 30 goaltenders, their actual GGVT, GVA, and calculated GGVT* in Appendix B. I am unable to explain why 28 of the 30 most extreme discrepancies occur in those 2 years.

If you compare observed GGVT to the distribution of GVA generated by average goalies, observed GGVT looks more like GVA than GVA itself. Of the 434 observed GGVT values, 31 fell above the average goalie 95% CI, 8 fell below the average goalie 95% CI, and 405 fell within the average goalie 95% CI. For GVA, the numbers were 24 above, 41 below, and 369 within. Thus more of the actual GGVT values fell into the average goalie GVA confidence interval than did actual GVA values.

55 goaltenders have a GVA > actual GGVT. 2 goaltenders, Bryzgalov in 2009 and Halak in 2009 had a GVA that was above the average goalie CI while their actual GGVT was within the average goalie CI.

Why is GGVT the same as GVA? GGVT is defined relative to threshold save percentages while GVA is defined relative to average save percentages. Comparing the values:

 Even Strength Penalty Kill Power Play Overall NHL Average 0.917 0.868 0.914 0.909 Calculated Threshold 0.914 0.863 0.911 0.905 Required Shots 80% Power 134930 73094 139262 82757

Sample sizes for binomial distribution testing are reasonably easy to calculate. For a given pair of probabilities we can calculate the number of trials needed to see a difference in the two probabilities. Imagine we had a group of "average" goalies and a group of "threshold" goalies. For p1=0.917 and p2=0.914, to have an 80% probability of seeing a difference, the required number of shots each group would need to face calculates as 134930. The table above shows the results for the other pairs of probabilities.

Conclusion

GGVT is statistically indistinguishable from GVA. The threshold save percentages used to calculate GVT are statistically indistinguishable from the average save percentages used to calculate GVA.

Acknowledgments

Again, I want to thank Sunny Mehta for his very helpful correspondence. I also want to thank Tom Awad again for his patient and thoughtful discussions of GVT.

Appendix A

GGVT: GGVT from Tom's tables

GVA: GVA calculated as before

StdDev: standard deviation of GVA derived from the simulations

Upper CI: Upper limit of the 95% CI for GVA

Lower CI: Lower limit of the 95% CI for GVA

 Year Name GGVT GVA StdDev Upper CI Lower CI 2009 AndersonCraig 18.50 19.36 13.73 46.27 -7.55 2009 AuldAlex -5.60 -8.99 7.11 4.94 -22.92 2009 BacklundJohan 0.20 0.14 1.39 2.87 -2.59 2009 BackstromNiklas -3.50 -9.75 11.65 13.09 -32.59 2009 BernierJonathan 3.90 4.55 2.77 9.98 -0.88 2009 BironMartin -7.20 -8.81 8.41 7.68 -25.30 2009 BoucherBrian -3.40 -7.57 8.09 8.29 -23.43 2009 BrodeurMartin 13.70 11.51 12.92 36.84 -13.82 2009 BrodeurMike 4.20 5.20 2.70 10.48 -0.08 2009 BryzgalovIlya 23.60 25.44 12.81 50.55 0.33 2009 BudajPeter 3.30 2.76 5.63 13.80 -8.28 2009 ClemmensenScott 2.40 1.43 7.41 15.96 -13.10 2009 ClimieMatt -1.20 -1.74 1.71 1.62 -5.10 2009 ConklinTy 10.00 11.26 8.03 27.01 -4.49 2009 CrawfordCorey 0.20 0.15 1.69 3.47 -3.17 2009 CurryJohn -3.00 -2.92 0.99 -0.97 -4.87 2009 DanisYann 2.50 2.58 4.12 10.65 -5.49 2009 DeslauriersJeff -7.50 -12.20 11.21 9.77 -34.17 2009 DipietroRick -0.80 -2.37 4.01 5.48 -10.22 2009 DubielewiczWade -1.40 -1.73 1.72 1.64 -5.10 2009 DubnykDevan -8.10 -10.38 6.87 3.09 -23.85 2009 DuchesneJeremy -0.50 -0.62 0.59 0.53 -1.77 2009 ElliottBrian 3.70 2.12 10.86 23.41 -19.17 2009 EllisDan 1.10 0.00 8.35 16.36 -16.36 2009 EmeryRay 0.40 -1.83 8.10 14.05 -17.71 2009 EnrothJhonas -0.50 -0.67 1.77 2.80 -4.14 2009 ErsbergErik 0.10 -0.01 4.48 8.77 -8.79 2009 FleuryMarc-Andre 0.00 -4.12 12.22 19.83 -28.07 2009 GaronMathieu -1.40 -3.62 8.45 12.94 -20.18 2009 GiguereJean-Sebastien 0.50 -0.36 9.24 17.75 -18.47 2009 GreissThomas 2.10 1.33 5.74 12.59 -9.93 2009 GustavssonJonas -4.10 -7.24 9.81 11.99 -26.47 2009 HalakJaroslav 20.20 21.08 10.70 42.06 0.10 2009 HardingJosh -1.10 -3.05 7.56 11.76 -17.86 2009 HedbergJohan 10.10 10.36 10.53 31.00 -10.28 2009 HillerJonas 18.30 18.48 12.33 42.64 -5.68 2009 HowardJimmy 24.70 27.12 12.27 51.17 3.07 2009 HuetCristobal -10.20 -14.60 9.40 3.82 -33.02 2009 JohnsonBrent 0.10 -1.49 6.63 11.51 -14.49 2009 JohnsonChad 1.40 1.20 3.32 7.71 -5.31 2009 KhabibulinNikolai 1.00 0.10 7.09 14.00 -13.80 2009 KhudobinAnton 2.80 3.58 2.02 7.54 -0.38 2009 KiprusoffMiikka 23.00 25.08 12.97 50.50 -0.34 2009 LaBarberaJason 8.30 9.65 6.30 22.00 -2.70 2009 LalimePatrick 0.20 0.13 5.98 11.85 -11.59 2009 LeclairePascal -12.50 -15.85 8.37 0.55 -32.25 2009 LegaceManny 0.90 -1.35 7.83 14.00 -16.70 2009 LehtonenKari 1.30 1.26 5.38 11.81 -9.29 2009 LeightonMichael 0.10 -2.48 8.67 14.51 -19.47 2009 LundqvistHenrik 25.70 24.55 13.05 50.13 -1.03 2009 LuongoRoberto 11.10 9.03 12.58 33.68 -15.62 2009 MacDonaldJoey -1.90 -2.43 3.64 4.71 -9.57 2009 MasonChris 11.00 9.60 11.92 32.97 -13.77 2009 MasonSteve -5.60 -10.88 11.91 12.46 -34.22 2009 McElhinneyCurtis -1.50 -1.98 6.61 10.99 -14.95 2009 MillerRyan 36.70 40.12 13.10 65.80 14.44 2009 NabokovEvgeni 27.90 30.38 13.64 57.11 3.65 2009 NeuvirthMichal 2.90 2.10 6.11 14.08 -9.88 2009 NiemiAntti 4.30 4.03 8.88 21.44 -13.38 2009 NiittymakiAntero 3.30 3.34 10.80 24.51 -17.83 2009 OsgoodChris -8.70 -12.33 6.83 1.06 -25.72 2009 PavelecOndrej -0.50 -2.10 10.65 18.77 -22.97 2009 PechurskiAlexander 0.20 0.23 1.05 2.29 -1.83 2009 PetersJustin -0.20 -1.21 4.41 7.42 -9.84 2009 PriceCarey 6.10 4.99 10.31 25.21 -15.23 2009 QuickJonathan 1.30 -2.89 12.81 22.22 -28.00 2009 RaskTuukka 24.10 26.92 10.02 46.57 7.27 2009 RaycroftAndrew 2.00 1.18 5.92 12.78 -10.42 2009 RinnePekka 4.70 2.33 11.23 24.35 -19.69 2009 RolosonDwayne 0.00 -3.19 11.37 19.10 -25.48 2009 SalakAlexander -1.90 -2.03 1.87 1.64 -5.70 2009 SchneiderCory 0.40 0.25 2.20 4.55 -4.05 2009 SmithMike -5.60 -10.64 9.76 8.50 -29.78 2009 TheodoreJose 4.80 3.98 10.80 25.14 -17.18 2009 ThomasTim 7.50 7.10 9.94 26.58 -12.38 2009 TokarskiDustin -1.20 -1.62 1.11 0.56 -3.80 2009 ToskalaVesa -16.90 -21.37 8.08 -5.54 -37.20 2009 TurcoMarty 7.60 7.93 11.51 30.48 -14.62 2009 ValiquetteSteve -5.60 -7.05 3.28 -0.62 -13.48 2009 VarlamovSemyon 2.30 0.08 7.70 15.16 -15.00 2009 VokounTomas 29.30 32.15 13.14 57.90 6.40 2009 WardCam 10.80 11.49 10.85 32.75 -9.77 2009 ZabaMatt -0.40 -0.38 1.22 2.00 -2.76

Appendix B

GGVT: GGVT from Tom's tables

GVA: GVA calculated as before

GGVT*: GGVT calculated as before

Defference: GGVT - GVA

 Year Name GGVT GVA GGVT* Difference 2005 GerberMartin 11.80 -10.40 -3.64 22.20 2005 KhabibulinNikolai -7.40 -26.29 -16.15 18.89 2005 GaronMathieu -2.20 -19.80 -10.31 17.60 2005 TheodoreJose -11.80 -28.17 -18.13 16.37 2005 GrahameJohn -9.60 -24.79 -14.91 15.19 2005 AuldAlex 11.40 -3.41 2.55 14.81 2005 TurcoMarty 5.10 -8.91 -2.35 14.01 2005 DipietroRick 5.50 -8.37 -1.58 13.87 2006 ThomasTim 8.70 -4.96 1.30 13.66 2005 DenisMarc 5.70 -7.09 -1.35 12.79 2005 AebischerDavid 2.60 -9.91 -3.61 12.51 2006 RaycroftAndrew -7.60 -20.08 -10.02 12.48 2005 MarkkanenJussi -9.80 -22.27 -14.42 12.47 2005 FleuryMarc-Andre 3.60 -8.70 -2.60 12.30 2005 LuongoRoberto 35.00 22.76 23.57 12.24 2005 NabokovEvgeni -10.60 -22.39 -13.78 11.79 2006 WardCam -3.00 -14.79 -6.95 11.79 2005 JosephCurtis 9.20 -2.47 2.62 11.67 2006 BelfourEd 4.60 -7.04 -1.30 11.64 2008 TurcoMarty -7.80 -19.33 -9.46 11.53 2005 KolzigOlie 2.80 -8.43 -0.92 11.23 2006 FleuryMarc-Andre 10.00 -1.22 4.10 11.22 2006 NiittymakiAntero -6.70 -17.90 -9.35 11.20 2005 BelfourEd -3.20 -14.38 -6.83 11.18 2005 NiittymakiAntero -0.70 -11.74 -5.47 11.04 2006 DenisMarc -16.10 -26.84 -17.47 10.74 2006 JosephCurtis -7.30 -17.90 -9.60 10.60 2005 RaycroftAndrew -10.40 -20.91 -13.57 10.51 2008 ToskalaVesa -16.10 -26.37 -15.95 10.27