The Puck Stops Here

Goaltending Goals Versus Threshold

In my recent sabermetrics and hockey posts, I have been discussing Tom Awad’s goals versus threshold system.  I have discussed the offensive portion of this system and today will move on to discussing the goaltending portion of this system.

A goaltender’s job is to stop shots on goal.  This is how they are evaluated here.  They are not evaluated by other stats such as wins or shutouts which have a much larger dependence upon the team in which the goalie plays.

The goaltending portion of the goals versus threshold system is calculated as below.  First the threshold level needs to be computed by the following formulas:

SPa = sum (SVg) / sum(SFg)

SPa is the average saves percentage in the NHL in a given season and SVg and SFg are the saves and shots that each individual goalie faced.

The threshold saves percentage SPt is then calculated using:

SPt = SPa - GAAI * GTV

Here GAAI is the league average goals against average and GTV is the goaltending threshold value, which is arbitrarily set to 0.04.  This means that the threshold level goalie will allow 4% more goals than the average goalie.

The goaltending goals versus threshold raw value GGVTraw is then calculated using:

GGVTraw = SV - (SF * SPa)

This calculates the extra number of saves that a goalie has versus threshold.  Here SV is the number of saves for an individual goalie, SF is a goalie’s shots faced and SPa is the league average saves percentage as calculated above.

The final calculation is an attempt to remove the team defence influence from a goaltender’s saves percentage.  This calculation is quite approximate and arbitrary.  It is assumed (with some attempt to statistically back things up) that a goalie is responsible for 75% of his saves and the other 25% comes from defence limiting shot quality.  Thus the final goalie goals versus threshold GGVT is calculated using:

GGVT = GGVTraw * 0.75

This system tends to leave the goals versus threshold for goalies higher than that of any position player.  While goaltending is usually the most important position on a given team, I do not accept that the best players in this or any system should be goaltenders in every season.  Also, this system is approximate at best and it does not include any contribution a goalie makes other than stopping shots.  For example, a goaltender with good stick handling skills can prevent shots by breaking up dump-ins and create breakouts with quick outlet passes.  The biggest problem with this system, however, comes from the arbitrariness of the 0.75 and SPa numbers.

In the future I will look at the leaders in this system and try to get a handle on how well it picks out the best goalies in the NHL.

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