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08-09-2012, 12:59 PM
Czech Your Math
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Originally Posted by seventieslord View Post
No, it's not necessarily luck-driven for the players doing the shooting and making the saves, but it does create a luck variable in +/- for everyone else on the ice. Goalie stands on his head for a shift, players who otherwise deserved a plus don't get one. Goalie lets in a stinker, players who didn't get a minus all get one, not to mention, the other team gets pluses they don't deserve.

PDO shows who was the beneficiary of more "puck luck" throughout the season. The swings between some players are surprising, and yes, they do appear to be fairly random. All other factors being equal, two players could have CORSIs on opposite ends of the spectrum but end up with the same +/- due to just PDO. One badly outperformed the other one but +/- doesn't catch it. CORSI does, and PDO explains the disconnect between CORSI and PDO. Their CORSI scores are very indicative of future performance, while the PDOs are random, so the player with the high CORSI is the much better bet going forward, when the luck starts to balance out or regress to the mean in the long term.
But who relies on simple plus-minus, esp. over a full season or less, as a supreme measure of value? At most, one might use simple +/- as a supplemental stat over a full season or more. Anyone serious would use adjusted plus-minus, preferably over multiple seasons, as one of the metrics of value. On a team level, I would use GF/GA ratio or differential. One doesn't need PDO to know that a large change will usually regress towards the previous level the following year, Bill James proved that decades ago for baseball.

I think there's too much put into stats like Corsi too. It's at least measuring something, but outshooting the opposition does not equal success. Outshoot Hasek, all you want, it might not help one bit. I think Corsi might tell you something about possession, but much less about overall effectiveness. Boston was outshot two years ago, but they won the Cup. I think they'll take the goals and leave the shots for their opponents.

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