Monday, October 12, 2009

Situation-Adjusted Save Percentage

My conclusion from a recent look at special teams performance was that a goalie's play while his team is killing a penalty is important for evaluation, but that his performance when his team was on the power play was not. The latter conclusion was based on the lack of correlation between EV SV% and PP SV%, which suggests that the results are either highly influenced by luck or the rest of the team.

Gabe Desjardins over at Puck Prospectus agrees with me, and has a nice article up with a more advanced look at evaluating goalies based on only their EV and PK play. His method looks like a solid one for comprehensive goalie evaluation, but I thought of developing a much simpler formula that can be used as a quick-and-dirty way to take special teams into account.

Over the last decade, 76.3% of the shots have come at EV, 3.7% on the PP, and 20.0% on the PK. That number is skewed up slightly by the 2005-06 season, as in both of the last two seasons 19.7% of the shots taken were by a team on the power play. If we ignore the PP shots, a good approximation of the average EV/PK split is 80/20. By assigning an 80% weighting to the goalie's EV SV% and a 20% weighting to his PK SV% we can quickly adjust for special teams factors.

This adjustment doesn't actually make much of a difference for most goalies, but it does impact goalies who either faced a disproportionate number of shots on the penalty kill or for whatever reason allowed a unusual number of shorthanded goals. Here are the top 20 in situation-adjusted save percentage since the lockout (min. 100 GP):

1. Niklas Backstrom, .923
2. Tomas Vokoun, .922
3. Roberto Luongo, .920
4. Tim Thomas, .919
5. Cristobal Huet, .918
5. Henrik Lundqvist, .918
5. Dominik Hasek, .918
8. Martin Brodeur, .917
8. J.S. Giguere, .917
10. Miikka Kiprusoff, .914
10. Chris Mason, .914
12. Manny Fernandez, .913
13. Martin Biron, .912
13. Ryan Miller, .912
13. Kari Lehtonen, .912
16. Ilja Bryzgalov, .910
16. Marc-Andre Fleury, .910
16. Ray Emery, .910
19. Rick Dipietro, 909
19. Dwayne Roloson, .909
19. Manny Legace, .909

I think the best use of this adjustment would be as a quick check when comparing goalies. Let's say you were voting on the 2009 Calder Trophy and you wanted to compare the performances of Steve Mason and Pekka Rinne. If you look at the raw save percentages it was pretty close, with Mason at .916 and Rinne at .917. What that doesn't show, however, is that Rinne faced an unusually low number of shots against on the penalty kill. If we multiply their EV SV% by 80% and their PK SV% by 20%, Mason edges ahead .917 to .914.

The formula also adjusts for goalies who were lucky or unlucky with shorthanded scoring chances against. Henrik Lundqvist and Cam Ward both had .916 save percentages last season. However, Lundqvist allowed 11 shorthanded goals against compared to Ward's 5, as the Rangers allowed a lot of shots and presumably a lot of scoring chances against on the power play. If we look at EV and PK play only, Lundqvist jumps to .919 while Ward falls to .915.

To simply adjust for special teams factors remember the "80/20 rule", and you'll be able to pick out the goalies who have the burden or good fortune of facing heavy or light work on the penalty kill.

3 comments:

Scott Reynolds said...

Recently Gabe has taken to expressing save percentage in a manner that includes missed shots as well as shots on goal. Given that save percentage is such an important statistic in our evaluation of goaltenders I was curious what you think about that swtich.

The Contrarian Goaltender said...

Yeah, I noticed he had done that. I'm not sure missed shots mean much, they probably even out for most goalies over a larger sample size, so I don't see it as a problem conceptually to include missed shots in the totals. However, I would be a bit leery of arenas being more likely to overcount or undercount missed shots, as I imagine counting missed shots would require more subjectivity than counting actual shots. Do you know if he has looked into scorer bias for missed shots?

The Collector said...

I want to know how it is you can make this post and not claim that you are plugging Sieve M8son. Even last year he was average at best, now look at him, Dan Cloutier high on crack could outplay him!