by PuckStopsHere on 07/08/09 at 02:06 AM ET
In this summer’s look at sabermetrics and hockey I have began by looking at +/- ratings and the results of a couple standard methods of their adjustment. +/- ratings are sometimes considered a problematic starting point. One problem is “signal to noise” there may be slightly over 100 goals scored in even strength for a given player in a given season. Their +/- is the difference between the goals for and the goals against. If there are a few fluke events they can carry the majority of the signal (i.e. if a player is on for a few fluke goals those goals can greatly influence their +/-). One way to try to get around this is to increase the number of events we are looking at. Buffalo Sabres goaltender coach Jim Corsi has pioneered another metric. Why not keep track of all even strength shots directed at the net both for and against a team when a player is on the ice. This includes goals scored, shots on goal, blocked shots and missed shots. The NHL keeps enough data in its games online that this can be calculated for each game. The main benefit of this method is that it gives roughly sixteen times the number of events for a player that standard +/- does. This is intended to increase the signal to noise (in experimental physics any counting number has an experimental error that scales with its square root so in principle the Corsi Number could be four times better).
The problem is that it equates a goal scored with a blocked shot. Those events are clearly not equivalent. The hope is that over a season everything will wash out. The same team that allows a lot of goals also allows a lot of failed shots directed at the goal. The hope is that the distinction between players is a real effect and not one of choice (i.e. a team might choose to allow a lot of low percentage long shots while another team might not). This number can be adjusted and interpreted in the same (or slightly different) manner to +/-. I will take a look at the results of this in the upcoming little while.
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