## Why goalies should almost never start back-to-back games

Tom Szczerbowski-USA TODAY Sports

Ilya Bryzgalov has seen a heavy workload this season. He should see less, because it doesn't make much sense when goalies play back-to-back games.

Kurt wrote an excellent article a couple of days ago noting that goalies playing on a second consecutive night have a save percentage much lower than goalies who have at least one off day between games. Goalies playing for the second consecutive game had an average save percentage of .892, versus .912 if they had at least one day off.

There was an interesting question in the comments that seemed to go overlooked: Is that all because of goalie fatigue, or is some of it because the skaters are fatigued and the goalie is facing tougher shots as a result?

To answer that, I did a similar study but restricting things to games where a team was on the second half of a back-to-back, so that we could compare the instances where they started a goalie who played in the game before to the instances where they started a rested goalie (data through Thursday's games):

 Team playing back-to-back Games started Saves made Shots faced Save percentage Goalie played previous game 97 2340 2624 .892 Goalie rested previous game 106 2719 2982 .912

Rested goalies playing behind a tired team had the exact same .912 save percentage as Kurt found overall for rested goalies -- which presumably means that rested goalies behind a rested team were also at .912, and that the team's rest has no impact on the goalie's save percentage. The entire 0.020 difference would then be due to goalie fatigue.

To put that 0.020 difference in perspective: Tim Thomas' career save percentage is .921; Michael Leighton's is .901. This difference is enormous, and would basically mean that no goalie should ever start back-to-back games.

However, the sample size is not large enough to say with confidence that the difference will be 0.020 in the long run. Extreme results are observed over small samples; You're a lot more likely to get heads on 80 percent of five coin flips than on 80 percent of five hundred flips. I'm pretty confident that the effect is real -- the sample size is large enough that the chances of observing a difference this large just by random luck is about a half a percent -- but we need more data to say with confidence how large the effect is.

So let's add in data from the 2011-12 season to make the sample larger:

 Team playing back-to-back Games started Saves made Shots faced Save percentage 2013 tired goalies 97 2340 2624 .892 2013 rested goalies 106 2719 2982 .912 2011-12 tired goalies 186 4843 5344 .907 2011-12 rested goalies 229 6291 6894 .913 Total for tired goalies 283 7183 7968 .901 Total for rested goalies 335 9010 9876 .912

With the larger sample size, we can say with reasonable confidence that a rested goalie stops about 1 percent more shots than a tired goalie, even when both of them are playing in front of a team that played on the previous day. It's not quite the difference between Thomas and Leighton, but it is still quite significant -- the difference between Lundqvist and Theodore or between Price and Garon.

And remember, the goalies who are most likely to be starting back-to-back games are the best players and the durable guys who feel great after a game. Even with that sample bias where our data puts more weight on the guys who are most likely to do well in the second game, we still see a very large effect.

When you add in the possibility of increased susceptibility to injury resulting from playing on back to back days, there is a strong case to be made that goalies starting back to back games should be a relatively rare occurrence.

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