by PuckStopsHere on 09/10/11 at 02:06 PM ET
In my recent discussion of the point share system as developed by Justin Kubatko of hockey-reference.com, I have discussed the offensive portion and the defensive portion. The part missing is the goaltender’s portion, so I will discuss that today.
The point share system attempts to credit the points teams gather from wins and (regulation) ties to individual players similar to the win shares system Bill James has developed in baseball. This system is necessarily somewhat approximate as ideal statistics have not been recorded throughout the history of the NHL and because their interpretation to provide individual player value is not fully understood.
The offensive system provides reasonable results though it runs into problems because of the arbitrary valuation of assists relative to goals. The defensive system is less reliable. It selects defencemen who play on good defensive teams and get top +/- ratings. This could just as easily select offensive defencemen as defensive ones. The problem is that defense is an inherently team activity and the point share system essentially shares it between all players on the ice. The goaltending system is an extension of this idea. It shares the portion marginal goals prevented to the goaltender that was not shared to the position players. The portions shared should be a function of the teams involved and their systems and players and not a constant share for all teams.
The goaltender portion is found as 2/7 of the marginal goals prevented by a team. It is 2/7 because it is assumed that all players contribute one part of their play to offence and one part to defence except for goalies who contribute both parts of their play to defence. This leaves 7 total parts for defence and two provided by goaltending.
First we calculate the shots against adjustment for goalies. This is (shots against per minute) / (league average shots against per minute). This is used to calculate a marginal goals against for each goalie using (1 + 7/12) * shots against adjustment * minutes played * league goals against per minute - goals against.
Then marginal goals per point are calculated as league goals / league points. The point shares for goalies are 2/7 * marginal goals against / marginal goals per point.
This calculation can only be carried out from 1983/84 onward when shot data has been recorded. This we cannot calculate a shots adjustment for goalies. This actor is assumed to be one in the era before recorded shots.
In the future I want to look at some of the results that this system provides in an effort to see how reliable it is.
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